Purpose: Ipilimumab, a monoclonal antibody that blocks cytotoxic T-lymphocyte-associated protein-4 interactions, enhances T-cell activation and promotes tumor immunity. This phase II study evaluated the safety and efficacy of ipilimumab monotherapy versus best supportive care (BSC) among patients with advanced/metastatic gastric or gastroesophageal junction cancer who achieved at least stable disease with first-line chemotherapy.Experimental Design: Eligible patients were randomized to ipilimumab 10 mg/kg every 3 weeks for four doses, then 10 mg/kg every 12 weeks for up to 3 years, or BSC, which could include continuation of fluoropyrimidine until progression or toxicity. The primary endpoint was immune-related progression-free survival (irPFS); secondary endpoints included PFS by modified World Health Organization criteria and overall survival (OS).Results: Of 143 patients screened, 57 were randomized to each arm. irPFS with ipilimumab versus BSC was not improved [2.92 months, 95% confidence interval (CI), 1.61-5.16 vs. 4.90 months, 95% CI, 3.45-6.54, HR = 1.44; 80% CI, 1.09-1.91; P = 0.097], resulting in study cessation. At study closeout, which occurred 8 months after the interim analysis, the median OS durations were 12.7 months (95% CI, 10.5-18.9) and 12.1 months (95% CI, 9.3-not estimable), respectively. Grade 3/4 treatment-related adverse events occurred in 23% of ipilimumab-treated patients, in whom diarrhea (9%) and fatigue (5%) were most frequent, and in 9% of active BSC-treated patients.Conclusions: Although ipilimumab at 10 mg/kg was manageable, it did not improve irPFS versus BSC. However, comparable median OS of approximately 1 year and a favorable safety profile support the investigation of ipilimumab in combination with other therapies for advanced gastric cancer. Clin Cancer Res; 23(19); 5671-8. ©2017 AACR.