Patients with relapsed small-cell lung cancer (SCLC) have few treatment options and dismal survival. Phase I/II data show activity of nivolumab in previously treated SCLC. CheckMate 331 is a randomized, open-label, phase III trial of nivolumab versus standard chemotherapy in relapsed SCLC. Patients with relapse after first-line, platinum-based chemotherapy were randomized 1 : 1 to nivolumab 240 mg every 2 weeks or chemotherapy (topotecan or amrubicin) until progression or unacceptable toxicity. Primary endpoint was overall survival (OS). Overall, 284 patients were randomized to nivolumab and 285 to chemotherapy. Minimum follow-up was 15.8 months. No significant improvement in OS was seen with nivolumab versus chemotherapy median OS, 7.5 versus 8.4 months; hazard ratio (HR), 0.86; 95% confidence interval (CI), 0.72-1.04; P = 0.11. A survival benefit with nivolumab was suggested in patients with baseline lactate dehydrogenase ≤ upper limit of normal and in those without baseline liver metastases. OS (nivolumab versus chemotherapy) was similar in patients with programmed death-ligand 1 combined positive score ≥1% versus <1%. Median progression-free survival was 1.4 versus 3.8 months (HR, 1.41; 95% CI, 1.18-1.69). Objective response rate was 13.7% versus 16.5% (odds ratio, 0.80; 95% CI, 0.50-1.27); median duration of response was 8.3 versus 4.5 months. Rates of grade 3 or 4 treatment-related adverse events were 13.8% versus 73.2%. Nivolumab did not improve survival versus chemotherapy in relapsed SCLC. No new safety signals were seen. In exploratory analyses, select baseline characteristics were associated with improved OS for nivolumab. •The primary endpoint of OS with nivolumab versus chemotherapy as second-line treatment of SCLC was not met.•Crossing of the survival curves indicates higher long-term survival with nivolumab in a subset of patients.•Post hoc analyses suggest patients with baseline LDH ≤ ULN and those without liver metastases may benefit from nivolumab.•The safety profile of nivolumab was consistent with prior studies and more favorable than that of chemotherapy.