9584 Background: In both previous 18- and 24-month follow-up reports from CheckMate 238, NIVO demonstrated significantly longer recurrence-free survival (RFS) than ipilimumab (IPI) in patients (pts) with resected stage IIIB/C or stage IV melanoma. Here we provide a more comprehensive analysis of treatment-related adverse events (TRAEs) for NIVO over discrete follow-up intervals and an investigation of the association of these AEs with efficacy (RFS). Methods: Eligible pts were aged ≥15 years and underwent complete resection of stage IIIB/C or IV melanoma. A total of 453 pts were treated with NIVO 3 mg/kg Q2W for up to 1 year. The primary endpoint was RFS. Pts were followed for safety for up to 100 days following their last dose; as of the previous 18-month database lock, all pts had been off study drug for > 100 days. Here safety data were analyzed within discrete time intervals: months 0–3 of treatment (0–3), months 3–12 of treatment (3–12), and from last dose to 100 days after last dose (+100). In addition, the association of TRAEs with RFS was investigated using the 24-month efficacy dataset, accounting for time-delay bias within the first 12 weeks after randomization. Results: The incidence of the first onset of TRAEs reported in ≥5% of pts was highest in the 0–3 time frame; the most common TRAEs with NIVO were fatigue (28% for 0–3 vs 6% for 3–12 vs 2% for +100), pruritus (16% vs 7% vs 1%), and diarrhea (15% vs 7% vs 2%). Most TRAEs with NIVO resolved within 3 months of occurrence, except for endocrine AEs, which could have required hormone supplementation, and skin AEs (median overall resolution time of 48 and 22 weeks, respectively). Similar results were observed in an analysis taking into account repeat occurrences of TRAEs over time. Analyses investigating the association of TRAEs with RFS are ongoing and will be presented. Conclusions: These results in pts with resected stage IIIB/C or IV melanoma are consistent with the established safety profile of NIVO. Based on the time periods analyzed, the majority of TRAEs with adjuvant NIVO occurred early during treatment, and patients had a reduced frequency of TRAEs after the treatment course. The majority of select TRAEs resolved within 3 months. Clinical trial information: NCT02388906.