OBJECTIVE To determine the safety and efficacy of treatment with gamma interferon (IFNgamma) in patients with metastatic carcinoid tumor. METHODS 51 patients were enrolled on this Phase II Eastern Cooperative Oncology Group (ECOG) study. Seventy five percent of them had hormonally active tumors. Treatment consisted of IFNgamma subcutaneously at a daily dose of 0.1 mg/m(2). Patents were evaluated for toxicity weekly for the first month and monthly thereafter; response was determined radiologically every 8 weeks. RESULTS Patients received treatment with IFNgamma for a median of 17.9 weeks (range 2-175). Toxicity was generally mild and expected: 61% experienced noninfected fever and 21% developed granulocytopenia. Three patients (6%) had a partial response; there were no complete responses. Median time to progression was 5.5 months (95% confidence interval 3.9-11.1). The 1-year progression free rate was 28% (13.4-43.4%). Median survival was 42 months, with a 1-year survival rate of 67% (53.3-80%). CONCLUSIONS This Phase II study demonstrated that therapy with IFNgamma in patients with metastatic carcinoid tumor was well-tolerated, but did not produce significant antitumor effects. The overall results were somewhat comparable to those previously seen with alpha interferons as well as cytotoxic drugs.