We tested the hypothesis that the combination of tremelimumab and interferon alfa-2b acting via different and possibly synergistic mechanisms would overcome tumor immune tolerance and lead to significant and durable clinical responses. We conducted a phase II study in which patients were administered tremelimumab 15 mg/kg/course (three cycles one cycle = 4 weeks) intravenously every 12 weeks. High-dose interferon alfa-2b (HDI) was administered concurrently, including intravenous induction at 20 MU/m2/d for 5 d/wk for 4 weeks followed by maintenance at 10 MU/m2/d subcutaneously three times a week for 8 weeks per course. From course 2 onward, HDI maintenance was administered subcutaneously. Thirty-seven patients with American Joint Committee on Cancer stage IV (9M1a, 6M1b, and 22M1c) were enrolled. Two patients had previously treated brain metastases. Grades 3 and 4 toxicities included neutropenia (six patients; 17%), diarrhea/colitis (four patients; 11%), liver enzyme increase (four patients; 11%), rash (four patients; 11%), fatigue (15 patients; 40%), and anxiety/depression (five patients; 14%). Response data were available for 35 patients. The best objective response rate (RR; Response Evaluation Criteria in Solid Tumors) by intention to treat was 24% (90% CI, 13% to 36%; four complete responses CRs and five partial responses PRs that lasted 6, 6, > 12, > 14, > 18, 20, > 28, 30, and > 37 months, respectively). Fourteen patients (38%) had stable disease (SD) that lasted 1.5 to 21 months. The median progression-free survival was 6.4 months (95% CI, 3.3 to 12.1 months). The median overall survival (OS) was 21 months (95% CI, 9.5 to not reached). There was a weak association between therapy-induced autoimmunity and clinical benefits (CR/PR/SD; P = .0059), baseline C-reactive protein (CRP) less than or equal to 2.7× the upper limit of normal and clinical benefits (P = .0494) and improved probability of survival (P = .0032), and baseline lymphocyte count of at least 1,000/μL and response (CR/PR; P = .0183) and clinical benefits (CR/PR/SD; P = .0255). Biomarker associations were not significant after adjustment for multiple comparisons. HDI can be administered combined with tremelimumab with acceptable toxicity and promising durable antitumor efficacy that warrant further testing in a randomized trial.