2655 Background: Associations between immune-related adverse events (irAEs) from checkpoint inhibitor therapy and outcomes have been previously evaluated, with most prior research finding a positive association between toxicity incidence and overall survival. This prior research has generally reported on more common tumor types. We use a unique data resource of a federally funded basket trial (NCT02834013) for patients with rare cancers to evaluate associations between irAEs and overall survival (OS) and progression-free survival (PFS) in a cohort of patients treated with combination checkpoint inhibitor therapy. Methods: We evaluated irAEs observed in the first cycle (6 weeks) of therapy that were possibly, probably, or definitely related to treatment; irAE grade was based on CTCAEv5. Patients received ipilimumab [1mg/kg intravenously (IV) every 6 weeks] plus nivolumab (240mg IV every 2 weeks). Landmarks methods were used to avoid immortal-time bias; PFS and OS were analyzed from day 43 with patients who died or progressed before that date excluded from analyses. Cox regression analyses were used to evaluate covariate associations. Results: We found that grade 1-2 treatment-related irAEs in the first cycle of therapy were associated with longer OS (multivariable hazard ratio, 95% confidence interval, p-value: 0.61, 0.49-0.75, p<0.001) compared to no treatment-related irAE in the first cycle, while grade 3-4 irAEs were associated with shorter OS (HR=1.41, 95% CI=1.04-1.90, p=0.025). Similar, but weaker, associations were observed with PFS, grade 1-2 treatment-related irAEs: HR=0.83, 95% CI=0.67-1.01, p=0.067 and grade 3-4: HR=1.35, 95% CI=1.02-1.78, p=0.037 compared to no treatment-related irAE in the first cycle. Grade 1-2 dermatologic toxicity were associated with improved OS compared to other grade 1-2 toxicities (HR=0.67, 95% CI=0.52-0.85, p=0.002). There were no significant differences between OS among patients with Grade 1-2 gastrointestinal, metabolic, hepatic, endocrine, and thyroid toxicities and fatigue and other Grade 1-2 toxicities. Conclusions: In this large cohort of patients with rare tumors receiving ipilimumab and nivolumab grade of irAE in the first cycle of therapy was prognostic for survival.