Data are limited for immunotherapy in patients with advanced NSCLC and poor performance status or other comorbidities. CheckMate 817 is a multi-cohort, open-label phase 3b/4 study investigating safety and efficacy of flat-dose nivolumab plus weight-based low-dose ipilimumab in advanced NSCLC. Here we evaluate this regimen as first-line treatment in special populations (cohort A1) and a reference population (cohort A; previously reported). Patients had previously untreated advanced NSCLC. Cohort A1 (n=198) had ECOG PS 2 or ECOG PS 0–1 with 1 of: asymptomatic untreated brain metastases, hepatic or renal impairment, or HIV. Cohort A (n=391) had ECOG PS 0–1. Patients with known EGFR mutations or ALK translocations sensitive to available targeted therapy were excluded from both cohorts. Nivolumab 240 mg Q2W plus ipilimumab 1 mg/kg Q6W was administered for two years or until disease progression/unacceptable toxicity. Safety and efficacy endpoints were assessed; cohort A1 analyses were exploratory. Cohort A1 patients were grouped as: ECOG PS 2 (n=139) and all other special populations (AOSP; n=59). Baseline characteristics were generally balanced between cohorts. Rates of grade 3–4 treatment-related adverse events (TRAEs) were similar between cohorts; within cohort A1, grade 3–4 TRAEs were numerically higher in AOSP versus the ECOG PS 2 subgroup; TRAEs leading to discontinuation were similar across populations (Table). ORR was 25% in cohort A1 (patients with ECOG PS 2, 20%; AOSP, 37%) and 35% in cohort A. PFS was numerically shorter in cohort A1 than cohort A; high TMB (≥10 mut/Mb) and higher PD-L1 expression (≥1% or ≥50%) were associated with numerically longer PFS in both cohorts (Table). First-line flat-dose nivolumab plus weight-based ipilimumab showed a consistent safety profile in special populations with advanced NSCLC, including those with ECOG PS 2. Patients with either high TMB or higher tumor PD-L1 expression appeared to exhibit improved efficacy.