Background: Consolidation PD-1/PD-L1 inhibition after chemoradiation (CRT) for unresectable stage III NSCLC improves overall survival. In stage IV NSCLC, the combination of nivolumab/ipilimumab improved overall survival compared to chemotherapy in patients with PD-L1 > 1% and performed favorably in patients with PD-L1 < 1%. The safety of consolidation nivolumab/ipilimumab after CRT has not been previously assessed. Methods: In this randomized, multi-center, phase II study, a total of 105 planned pts with unresectable stage IIIA/IIIB NSCLC will receive chemoradiation, then randomize 1:1 to either nivolumab 480mg IV q4 wks (Arm A) or nivolumab 3mg/kg IV q2 wks + ipilimumab 1mg/kg IV q6 wks (Arm B), for up to 24 wks. In this planned interim analysis, the safety of the first 50 patients, with 25 patients treated on each arm, is assessed. Results: From 9/2017 to 6/2019, the first 50 patients were accrued and analyzed for this planned safety analysis. Baseline characteristics for Arm A/B: median age 64/62, stage IIIA 17/16, stage IIIB 8/9, non-squamous 14/13, squamous 11/12. The median number of cycles completed in Arm A was 6 (range 1-6, cycle length q4 wks) and in Arm B was 4 (range 1-4, cycle length q6 wks). The rate of treatment-related adverse events leading to discontinuation of therapy was 16% in Arm A and 40% in Arm B. The percentage of patients with any > grade 3 adverse event (AE) was 32% in Arm A and 44% in Arm B. With respect to immune-related AE (irAEs), the percentage of patients with any ≥grade 2 was 44% in Arm A and 60% in Arm B; any ≥ grade 3 irAEs was 16% in Arm A and 32% in Arm B. The incidence of > grade 2 pneumonitis was 16% in Arm A and 36% in Arm B. The percentage of patients with > grade 3 pneumonitis was 4% in Arm A and 20% in Arm B. No treatment-related deaths were reported on either arm. Conclusions: In the post chemoradiation setting, the incidence of > grade 3 toxicity was greater in the consolidative nivolumab/ipilimumab arm, which resulted in a higher rate of treatment discontinuation than nivolumab alone. The Data and Safety Monitoring Board recommended continued enrollment without modification to the trial and the study currently remains open to accrual (66 of 105 patients have been enrolled as of 1/17/2020). Clinical trial information: NCT03285321