Background: Levodopa is the most effective symptomatic therapy for Parkinson's disease, but patients with advanced Parkinson's disease develop motor fluctuations with chronic oral levodopa therapy. Foslevodopa-foscarbidopa is a soluble formulation of levodopa and carbidopa prodrugs that is delivered as a 24-h/day continuous subcutaneous infusion, and we aimed to assess the safety and efficacy of this formulation in patients with advanced Parkinson's disease., Methods: A 12-week randomised, double-blind, double-dummy, active-controlled study was done at 65 academic and community study centres in the USA and Australia. Patients with levodopa-responsive advanced Parkinson's disease inadequately controlled on current therapy, including at least 2·5 h of average daily off time, were randomly assigned (1:1) to continuous subcutaneous infusion of foslevodopa-foscarbidopa plus oral placebo or to oral immediate-release levodopa-carbidopa plus continuous subcutaneous infusion of placebo solution. Randomisation was stratified by site by means of a permutated-block schedule with a block size of two. The participants, treating investigators, study site personnel, and sponsor were masked to treatment group allocation. The primary and first key secondary endpoint in the hierarchical testing strategy were change from baseline to week 12 in on time without troublesome dyskinesia and off time, respectively; both endpoints were evaluated by an intention-to-treat analysis applying a mixed model for repeated measures analysis. Safety and tolerability were assessed throughout the study. The study is completed and is listed on ClinicalTrials.gov, NCT04380142., Findings: Between Oct 19, 2020, and Sept 29, 2021, of 270 participants screened and 174 enrolled, 141 were randomly assigned and received continuous subcutaneous infusion of foslevodopa-foscarbidopa plus oral placebo capsules (n=74) or oral encapsulated immediate-release levodopa-carbidopa plus continuous subcutaneous infusion of placebo solution (n=67). Compared with levodopa-carbidopa, foslevodopa-foscarbidopa showed a significantly greater increase in on time without troublesome dyskinesia (model-based mean [SE] 2·72 [0·52] vs 0·97 [0·50] h; difference 1·75 h, 95% CI 0·46 to 3·05; p=0·0083) and a significantly greater reduction in off time (-2·75 [0·50] vs -0·96 [0·49] h; difference -1·79 h, -3·03 to -0·54; p=0·0054). Hierarchical testing ended after the first secondary endpoint. Adverse events were reported in 63 (85%) of 74 patients in the foslevodopa-foscarbidopa group versus 42 (63%) of 67 in the levodopa-carbidopa group, and incidences of serious adverse events were similar between the groups (six [8%] of 74 vs four [6%] of 67, respectively). The most frequent adverse events in the foslevodopa-foscarbidopa group were infusion site adverse events (erythema 20 [27%]), pain 19 [26%]), cellulitis (14 [19%]), and oedema (nine [12%]), most of which were non-serious and mild-moderate in severity. The only system organ class that had more than one serious adverse event in the foslevodopa-foscarbidopa group was infections and infestations (catheter site cellulitis [one [1%]] and infusion site cellulitis [one [1%]). Adverse events led to premature discontinuation of study drug in 16 (22%) of 74 participants in the foslevodopa-foscarbidopa group versus one (1%) of 67 participants in the oral levodopa-carbidopa group., Interpretation: Foslevodopa-foscarbidopa improved motor fluctuations, with benefits in both on time without troublesome dyskinesia and off time. Foslevodopa-foscarbidopa has a favourable benefit-risk profile and represents a potential non-surgical alternative for patients with advanced Parkinson's disease., Funding: AbbVie., Competing Interests: Declaration of interests MJS received honoraria or consultation fees from Allergan, Abbott, AbbVie, Acorda, Medtronic, Merz Therapeutics, Neurocrine Pharmaceuticals, Sunovion, and Teva. JA is a study investigator and has received honoraria from AbbVie, Allergan, Teva, US World Meds, Medtronic, and Abbot; he is an investigator in studies sponsored by AbbVie, Biogen, Acadia, Northwestern University, Neuroderm, Massachusetts General Hospital, and AstraZeneca. He is also a scientific advisor for AbbVie. VSCF receives a salary from NSW Health; has received unrestricted research grants from the Michael J Fox Foundation, AbbVie and Merz; has been on Advisory Boards for AbbVie, Allergan, Ipsen, Merz, Praxis, Seqirus, Stada, Teva, and UCB; he receives royalties from Health Press. TEK has received funding from Stada, AbbVie, UCB Pharma, Seqiris, and Teva. KK has served as a consultant for AbbVie and has participated in AbbVie speakers’ bureaus. IL is a member of the faculty of the University of California San Diego and is supported by an endowment and university funds; she also receives funds as Chief Editor of Frontiers in Neurology and as member of the Rossy PSP Toronto Center; she is an investigator in studies funded by Roche, AbbVie, Biogen, Centogene, EIP-Pharma, Biohaven Pharmaceuticals, Novartis, and United Biopharma SR–UCB; she receives funds and participates in studies from the National Institutes of Health, the Michael J Fox Foundation, the Parkinson Foundation, the Lewy Body Association, and CurePSP; she is a member of the Scientific Advisory Board for Amydis but does not receive funds. DO has received speaking honoraria from AbbVie. MAS has received consulting fees from Medtronic, and clinical trial funding from AbbVie, US WorldMeds, Sanofi, Abbott, Takeda, and Insightec. DGS is a member of the faculty of the University of Alabama at Birmingham; is supported by endowment and university funds; and is an investigator in studies funded by AbbVie, the American Parkinson Disease Association, the Michael J Fox Foundation for Parkinson Research, Alabama Department of Commerce, the Department of Defense, and NIH grants P50NS108675, R25NS079188, and T32NS095775; he has a clinical practice and is compensated for these activities through the University of Alabama Health Services Foundation; in addition, he has served as a consultant for or received honoraria from AbbVie, the Parkinson Study Group, Curium Pharma, Roche, Appello, the International Parkinson Disease and Movement Disorder Society, Theravance, McGraw Hill, Sanofi-Aventis, and Alnylam Pharmaceutics. EOV has received consulting fees from AbbVie, Acorda, Allergan, and Medtronic. RAH has served on a scientific advisory board for Inhibikase Therapeutics, Impel NeuroPharma, CereSpir, and Vivifi Biotech; he has received financial compensation for work as a consultant from AbbVie, Acadia Pharmaceuticals, Acorda Therapeutics, Adamas Pharmaceuticals, Affiris, Amneal Pharmaceuticals, ApoPharma, Aptinyx, AstraZeneca, Axovant Gene Therapies, Biomarin International, Biotie Therapies, Britannia Pharmaceuticals., Cadent Therapeutics, Cerevance, Cerevel Therapeutics, Curium, Cynapsus Therapeutics, Denali Therapeutics, Eli Lilly, Enterin, F Hoffmann-La Roche, GE Healthcare, Global Kinetics Business Consultants, Global Life Sciences Solutions, Impax Laboratories, Impel NeuroPharma, Inhibikase Therapeutics, Intec, Intrance Medical Systems, International Stem Cell Corporation (ISCO), Jazz Pharmaceuticals, Kashiv BioSciences, KeifeRx, Kyowa Kirin, Lundbeck, Merz Pharma, the Michael J Fox Foundation, Mitsubishi Tanabe Pharma America, the National Parkinson Foundation, Neuro Challenge Foundation for Parkinson's, Neurocrine Biosciences, NeuroDerm, Neuropore Therapies, Novus Biologicals, Orion, Parkinson Study Group, Perception OpCo (Cerevel Therapeutics), Pfizer, Pharma Two B, PharmaTher, Prexton Therapeutics, Regenera Pharma, Research Catalyst, Revance Therapeutics, Sanofi-Aventis, Sarepta Therapeutics, Scion NeuroStim, Seelos Therapeutics, Sio Gene Therapies, Sunovion Pharmaceuticals, Supernus Pharmaceuticals, Takeda Pharmaceuticals, Teva Pharmaceutical Industries, Tolmar Pharmaceuticals, UCB Biosciences, US WorldMeds, and Vivifi Biotech. He serves on speakers’ bureaus for AbbVie, Acorda Therapeutics, Adamas Pharmaceuticals, Amneal Pharmaceuticals, Kyowa Kirin, Lundbeck, Neurocrine Biosciences, Sunovion Pharmaceuticals, Teva Pharmaceuticals, and US WorldMeds. He has received research support from AbbVie, Adamas Pharmaceuticals, AstraZeneca, Atlantic Research–Chelsea Therapeutics, Axovant Sciences, Biogen, Biotie Therapies, Cavion, Centogene, Cerevance, Cerevel Therapeutics, Civitas Therapeutics, Cynapsus Therapeutics, Dart NeuroScience, Enterin, F Hoffman-La Roche, Global Kinetics Corporation, Impax Laboratories, Intec Pharma, Integrative Research Laboratories Sweden, Jazz Pharmaceuticals, Kyowa Kirin, Lundbeck, the Michael J Fox Foundation, National Institutes of Health, National Institute of Neurological Disorders and Stroke, Neuraly, NeuroDerm, Northwestern University, the Parkinson Study Group, Pfizer, Pharma Two B, Prexton Therapeutics, Revance Therapeutics, Sanofi US Services, Sun Pharma Advanced Research, Sunovion Pharmaceuticals, and UCB Biopharma; he holds stock in Axial Biotherapeutics and Inhibikase Therapeutics, and has received license fee payments from USF for Parkinson's Disease Diary. KB is a former employee of AbbVie, is currently employed by Harmony Biosciences, and might hold AbbVie stock or options. MFF, NF, AJ, WZR, ST, and HZ are employees of AbbVie and might hold stock or options., (Copyright © 2022 Elsevier Ltd. All rights reserved.)