Background: Hereditary angioedema (HAE) is a debilitating disorder resulting from C1-esterase inhibitor (C1-INH) deficiency. In the COMPACT phase 3 study the prophylactic use of a subcutaneous C1 inhibitor (C1-INH [SC], HAEGARDA ® , CSL Behring) twice weekly significantly reduced the frequency of acute edema attacks. Analysis of treatment effects by subgroups, onset of effect, and other exploratory analysis have not been reported., Methods: This is a post hoc exploratory analysis on data from the randomized, placebo-controlled COMPACT study. 90 patients with C1-INH-HAE were randomized to 1 of 4 treatment sequences: C1-INH (SC) 40 or 60 IU/kg of body weight twice weekly for 16 weeks, preceded or followed by a placebo period. The pre-specified primary efficacy endpoint was the time-normalized number of HAE attacks, and pre-specified secondary efficacy endpoints were the percentage of patients with a certain treatment response (≥ 50% reduction on C1-INH (SC) versus placebo in the time-normalized number of attacks) and the time-normalized number of use of rescue medication. Pre-specified exploratory endpoints included severity of attacks, alone and combined with rescue medication use. Post hoc analyses included exploration of onset of effect and clinical assessment of patients with < 50% of response., Results: Subgroup findings by various patient characteristics showed a consistent preventive effect of C1-INH (SC). In a post hoc analysis of attacks, the onset of the preventive effect within the first 2 weeks after treatment initiation in COMPACT showed that 10/43 patients (23%) experienced attacks of any severity with 60 IU/kg versus 34/42 patients (81%) with placebo. The need for rescue medication was tenfold lower with 60 IU/kg (35 treated attacks) versus placebo (358 treated attacks). A qualitative analysis of the 4 patients treated with 60 IU/kg and with < 50% reduction of attacks demonstrated a reduction in severity of attacks, rescue medication use, and symptom days which was considered a clinically meaningful treatment effect., Conclusions: C1-INH (SC) prophylaxis demonstrated a preventive treatment effect with evidence of benefit within 2 weeks. A consistent treatment effect at recommended C1-INH (SC) dosing was evident in all subgroups of patients with type I/II HAE and by various measures of disease and treatment burden. Trial registration EU Clinical Trials Register, 2013-000916-10, Registered 10 December 2013, https://www.clinicaltrialsregister.eu/ctr-search/trial/2013-000916-10; ClinicalTrials.gov Register, NCT01912456, Registered 31 July 2013, https://clinicaltrials.gov/ct2/show/NCT01912456., Competing Interests: Competing interestsHHL received institutional support from CSL Behring for the conduct of this study, and travel expenses and/or consultancy fees and speaker’s honoraria from CSL Behring, Shire/Dyax/ViroPharma, and Salix/Pharming. TC is a speaker for CSL Behring, Grifols and Dyax/Shire. He performs research for BioCryst, Boehringer Ingelheim, CSL Behring, Genentech, GlaxoSmithKline, Grifols, Merck, Novartis, Pharming, Sanofi, and Shire. He has received consultancy fees and/or speaker’s honoraria from BioCryst, Bellrose, CSL Behring, Dyax, Merck, Novartis, Pharming Technologies, and Shire, and has received non-financial support from CSL Behring, Shire, and Grifols. BZ reports grant support from the Department of Defense and consultancy fees from Alnylam, Arrowhead Pharmaceuticals, BioCryst Pharmaceuticals, Nektar, CSL Behring, and Shire, and led the Scientific Steering Committee for this study. HJL has received grant support from CSL Behring, consultancy fees, and speaker’s honoraria from CSL Behring, Pharming, and Shire, and travel support from CSL Behring. MC has received grants from Shire and personal fees from Alnylam, BioCryst Pharmaceuticals, CSL Behring, Dyax, KalVista Pharming Technologies, Shire, Sobi (Swedish Orphan Biovitrum), and ViroPharma. KB reports personal fees from CSL Behring and Shire, outside the submitted work. HB received consultancy fees and speaker’s honoraria from CSL Behring, Pharming, and Shire. WL reports grant support from BioCryst Pharmaceuticals, CSL Behring, and Shire/Viropharma/Dyax; consultancy fees paid to his institution from Adverum, BioCryst Pharmaceuticals, CSL Behring, Pharming Technologies, and Shire/Virophama/Dyax; speaker’s fees from Pharming Technologies, Shire/Viropharma and CSL Behring; and non-financial support from the US Hereditary Angioedema Association outside the submitted work. JB reports grant support and personal fees from BioCryst Pharmaceuticals, CSL Behring, and Shire, outside the submitted work. MM reports grant support and consultant/speaker’s fees from CSL Behring, Shire, Dyax, and Shire; and personal fees from Salix and Pharming Technologies, outside the submitted work. DL has served on the speaker’s bureau, as a consultant, on a steering committee, and as a clinical investigator for CSL Behring. MR has received research grants from BioCryst Pharmaceuticals, CSL Behring, Dyax, Pharming Technologies, and Shire; consultant fees from Adverum Biotechnologies, Alnylam Pharmaceuticals, BioCryst Pharmaceuticals, CSL Behring, Global Blood Therapeutics, Ionis Pharmaceuticals, KalVista Pharmaceuticals, Pharming Technologies, and Shire; speaker’s honoraria from CSL Behring, Shire, and Pharming; and is an uncompensated advisory board member for the US Hereditary Angioedema Association. TM, SP HF and IP are employees of CSL Behring.