BACKGROUND Approximately 20% of patients with non-small-cell lung cancer (NSCLC) receive a diagnosis of stage III disease. There is no current consensus regarding the most appropriate treatment for these patients. METHODS In this open-label, phase 2 trial, we randomly assigned patients with resectable stage IIIA or IIIB NSCLC to receive neoadjuvant nivolumab plus platinum-based chemotherapy (experimental group) or chemotherapy alone (control group), followed by surgery. Patients in the experimental group who had R0 resections received adjuvant treatment with nivolumab for 6 months. The primary end point was a pathological complete response (0% viable tumor in resected lung and lymph nodes). Secondary end points included progression-free survival and overall survival at 24 months and safety. RESULTS A total of 86 patients underwent randomization; 57 were assigned to the experimental group and 29 were assigned to the control group. A pathological complete response occurred in 37% of the patients in the experimental group and in 7% in the control group (relative risk, 5.34; 95% confidence interval [CI], 1.34 to 21.23; P=0.02). Surgery was performed in 93% of the patients in the experimental group and in 69% in the control group (relative risk, 1.35; 95% CI, 1.05 to 1.74). Kaplan-Meier estimates of progression-free survival at 24 months were 67.2% in the experimental group and 40.9% in the control group (hazard ratio for disease progression, disease recurrence, or death, 0.47; 95% CI, 0.25 to 0.88). Kaplan-Meier estimates of overall survival at 24 months were 85.0% in the experimental group and 63.6% in the control group (hazard ratio for death, 0.43; 95% CI, 0.19 to 0.98). Grade 3 or 4 adverse events occurred in 11 patients in the experimental group (19%; some patients had events of both grades) and 3 patients in the control group (10%). CONCLUSIONS In patients with resectable stage IIIA or IIIB NSCLC, perioperative treatment with nivolumab plus chemotherapy resulted in a higher percentage of patients with a pathological complete response and longer survival than chemotherapy alone.