Background: Stereotactic body radiotherapy (SBRT) is safe and effective for treatment of extracranial metastatic disease, but its safety when combined with immune checkpoint inhibitors (ICI) has not yet been comprehensively reported. Here we report adverse events (AEs) associated with combined SBRT and ICI using prospectively-collected data on patients in three trials investigating multi-site SBRT combined with ICI., Methods: Patients were included from three prospective trials of ICI (pembrolizumab; nivolumab/urelumab or nivolumab/cabiralizumab; nivolumab/ipilimumab) with SBRT to 1-4 sites. AEs were recorded prospectively using the CTCAE v4.0. Survival was analyzed using Kaplan-Meier method with a 90-day landmark. Association of patient characteristics with cumulative incidence of AEs was assessed using Fine-Gray regression., Results: 213 patients were included, with a median follow-up of 10 months. Over the follow-up period, 50 % and 27 % of patients experienced at least one grade ≥ 2 or grade ≥ 3 AE, respectively. Cumulative incidences of grade ≥ 2 and grade ≥ 3 AEs at 6 months were 47 % and 23 %, respectively. Three grade 5 AEs rated "possibly" related to treatment occurred outside the 90-day dose-limiting toxicity window. Landmarked survival analysis of patients with or without grade ≥ 3 AEs showed no significant difference in progression-free or overall survival. Dual-agent ICI was significantly associated with grade ≥ 3 AE., Conclusion: This analysis features the largest prospectively evaluated cohort of patients treated with combination ablative SBRT and ICI to date and provides context for future trial design. We conclude that multi-site SBRT and ICI can be safely co-administered when SBRT is delivered with prioritization of normal tissue constraints., Competing Interests: Declaration of Competing Interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Connor Lynch: none to declare. Mark C. Korpics: none to declare. Rohan R. Katipally: none to declare. Christine M. Bestvina: Institutional funding from AstraZeneca and Bristol Myers Squibb; consulting fees from AstraZeneca, Bristol Myers Squibb, CVS, Daiichi Sankyo, Genentech, Jazz, Johnson and Johnson, Novartis, Pfizer, Regeneron, Sanofi, Seattle Genetics, Takeda, Tempus; participates in the Data Safety Monitoring and/or Advisory Board of AstraZeneca, Bristol Myers Squibb, CVS, Daiichi Sankyo, Genentech, Jazz, Johnson and Johnson, Novartis, Pfizer, Regeneron, Sanofi, Seattle Genetics, Takeda, Tempus. Sean P. Pitroda: Speaker’s Bureau for AACR and Immunorad conferences; co-founder of PersonaDx; co-inventor of patent related to metastatic disease and prognostication via University of Chicago IP office. Jyoti D. Patel: Consulting and Advisory Board for Abbvie, AstraZeneca, Takeda Science Foundation, Genentech, Anheart Therapeutics; travel, accommodations, or expenses from Tempus. Jason J. Luke: DSMB: Abbvie, Agenus, Evaxion, Immutep, Shionogi; Scientific Advisory Board: (no stock) 7 Hills, Affivant, BioCytics, Bright Peak, Exo, Fstar, Inzen, RefleXion, Xilio (stock) Actym, Alphamab Oncology, Arch Oncology, Duke Street Bio, Elipscience, Kanaph, NeoTx, Onc.AI, OncoNano, physIQ, Pyxis, Saros, STipe, Tempest; Consultancy with compensation: Abbvie, Agenus, Alnylam, AstraZeneca, Askgene, Atomwise, Bayer, Bristol-Myers Squibb, Castle, Checkmate, Codiak, Crown, Cugene, Curadev, Day One, Eisai, EMD Serono, Endeavor, Flame, G1 Therapeutics, Genentech, Gilead, Glenmark, HotSpot, Kadmon, Ko Bio Labs, Krystal, KSQ, Janssen, Ikena, Inzen, Immatics, Immunocore, Incyte, Instil, IO Biotech, LegoChem, Macrogenics, Merck, Mersana, Nektar, Novartis, Partner, Pfizer, Pioneering Medicines, PsiOxus, Regeneron, Replimmune, Ribon, Roivant, Servier, STINGthera, Storm, Sumoitomo, Synlogic, Synthekine, Teva; Research Support: (all to institution) AbbVie, Astellas, Astrazeneca, Bristol-Myers Squibb, Corvus, Day One, EMD Serono, Fstar, Genmab, Hot Spot, Ikena, Immatics, Imugene, Incyte, Janux, Kadmon, KAHR, Macrogenics, Merck, Moderna, Nektar, Next Cure, Novartis, Numab, Palleon, Pfizer, Replimmune, Rubius, Servier, Scholar Rock, Synlogic, Takeda, Trishula, Tizona, Tscan, Werewolf, Xencor; Patents: US-11638728 (Microbiome Biomarkers for Anti-PD-1/PD-L1 Responsiveness: Diagnostic, Prognostic and Therapeutic Uses Thereof). Steven J. Chmura: Institutional funding from Bristol Myers Squibb; Family member employed by Astellas Pharmaceuticals. Aditya Juloori: Institutional funding from AstraZeneca; Consulting fees from Isoray, General Electric, and Aqualung., (Copyright © 2024 Elsevier Ltd. All rights reserved.)