Purpose: To determine better the activity of high-dose interieukin-2 (IL-2) either alone or in combination with interferon alfa-2b (IFN; Schering-Plough, Kenilworth, NJ) in patients with metastatic renal cell carcinoma, the IL-2 Working Group initiated a randomized phase II trial. Patients and Methods: Patients were randomly assigned to receive treatment with either IL-2 (Chiron Corp, Emery ville, CA) 1.33 mg/m2 (∼ 600,000 IU/kg) alone or IL-2 0.8 mg/m2 and IFN 3 × 106 U/m2 administered by bolus intravenous injection every 8 hours, days 1 to 5 and 15 to 19 (maximum, 28 doses). All patients had an Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 and normal organ function. After 28 patients were entered onto each arm, the IL-2/IFN arm was closed because of a failure to meet predetermined efficacy criteria. An additional 43 patients (total, 71 ) were assigned to receive IL-2 alone. Results: Toxicities were similar for both study arms. Hypotension requiring pressors was the most frequent dose-limiting toxicity. Only 11 of 99 patients experienced severe toxicity; there were no irreversible side effects or treatment-related deaths. Responses were seen in three of 28 patients (11%) on IL-2/IFN (three partial responses [PRs] lasting 14, 7, and 7 months) and 12 of 71 patients (17%) on IL-2 alone (four complete responses [CRs] and eight PRs). Six of the partial responders on IL-2 and two on IL-2/IFN experienced greater than 90% reduction in tumor mass. Ten of the 12 responders to IL-2 have ongoing responses of 12 + to 26 + months in duration. Conclusion: We conclude that both IL-2 and IL-2/IFN therapy have activity in metastatic renal cell carcinoma. In particular, therapy with high-dose IL-2 alone produces meaningful and durable responses with manageable and reversible toxicity. This study supports the contention that high-dose IL-2 represents the treatment of choice in selected patients with advanced renal cell carcinoma.