Background On the basis of data from a phase 2 trial that compared the checkpoint inhibitor ipilimumab at doses of 0.3 mg, 3 mg, and 10 mg per kilogram of body weight in patients with advanced melanoma, this phase 3 trial evaluated ipilimumab at a dose of 10 mg per kilogram in patients who had undergone complete resection of stage III melanoma. Methods After patients had undergone complete resection of stage III cutaneous melanoma, we randomly assigned them to receive ipilimumab at a dose of 10 mg per kilogram (475 patients) or placebo (476) every 3 weeks for four doses, then every 3 months for up to 3 years or until disease recurrence or an unacceptable level of toxic effects occurred. Recurrence-free survival was the primary end point. Secondary end points included overall survival, distant metastasis-free survival, and safety. Results At a median follow-up of 5.3 years, the 5-year rate of recurrence-free survival was 40.8% in the ipilimumab group, as compared with 30.3% in the placebo group (hazard ratio for recurrence or death, 0.76; 95% confidence interval [CI], 0.64 to 0.89; P