Standard treatment with neoadjuvant nivolumab plus chemotherapy significantly improves outcomes in patients with resectable non-small-cell lung cancer (NSCLC). Perioperative treatment (i.e., neoadjuvant therapy followed by surgery and adjuvant therapy) with nivolumab may further improve clinical outcomes.In this phase 3, randomized, double-blind trial, we assigned adults with resectable stage IIA to IIIB NSCLC to receive neoadjuvant nivolumab plus chemotherapy or neoadjuvant chemotherapy plus placebo every 3 weeks for 4 cycles, followed by surgery and adjuvant nivolumab or placebo every 4 weeks for 1 year. The primary outcome was event-free survival according to blinded independent review. Secondary outcomes were pathological complete response and major pathological response according to blinded independent review, overall survival, and safety.At this prespecified interim analysis (median follow-up, 25.4 months), the percentage of patients with 18-month event-free survival was 70.2% in the nivolumab group and 50.0% in the chemotherapy group (hazard ratio for disease progression or recurrence, abandoned surgery, or death, 0.58; 97.36% confidence interval [CI], 0.42 to 0.81; P<0.001). A pathological complete response occurred in 25.3% of the patients in the nivolumab group and in 4.7% of those in the chemotherapy group (odds ratio, 6.64; 95% CI, 3.40 to 12.97); a major pathological response occurred in 35.4% and 12.1%, respectively (odds ratio, 4.01; 95% CI, 2.48 to 6.49). Grade 3 or 4 treatment-related adverse events occurred in 32.5% of the patients in the nivolumab group and in 25.2% of those in the chemotherapy group.Perioperative treatment with nivolumab resulted in significantly longer event-free survival than chemotherapy in patients with resectable NSCLC. No new safety signals were observed. (Funded by Bristol Myers Squibb; CheckMate 77T ClinicalTrials.gov number, NCT04025879.).