OBJECTIVE A phase II trial of interleukin-2 (IL-2) and interferon alfa (IFN-alpha) in metastatic renal cell carcinoma (RCCa) was conducted. A lower dosage of IL-2 was given via continuous intravenous (IV) infusion, a route with documented tumor activity associated with less toxicity, with the purpose of improving the therapeutic index of this treatment in an outpatient setting. METHODS Thirty patients with metastatic RCCa were treated with the combination of IL-2 and IFN-alpha-2A. IL-2 was administered on days 1 through 4 of each treatment week, as a continuous IV infusion at a dose of 2 x 10(6) U/m2/d. IFN-alpha-2A was administered intramuscularly or subcutaneously on days 1 and 4 of each treatment week, at a dose of 6 x 10(6) U/m2/d. One treatment course included 4 weeks of treatment followed by a 2-week rest. Patients received therapy as outpatients except for the first 4 days of treatment, cycle 1 only. All patients were assessable for toxicity and response assessment. A total of 105 courses of therapy were administered, 51% at full dose. RESULTS Sixteen patients experienced toxicities resulting in dosage modification. The major treatment-limiting toxicities were gastrointestinal, neurologic, and fatigue. Nine patients (30%) had partial remissions (PRs) with a median duration of responses of 12+ months. The median time to response was 11 weeks. Two partial responders whose sites of metastatic disease were renal fossa and mediastinal lymph nodes (LN), respectively, were found to have achieved a pathologic complete remission (pCR) after surgery. A third patient with a pCR of axillary LN was rendered into a surgical complete remission (sCR) with salvage nephrectomy. Median survival of patients obtaining a PR has not been reached with a median follow-up time of 19+ months. CONCLUSIONS IL-2 and IFN-alpha-2A is well tolerated in the outpatient treatment setting and demonstrates significant clinical activity against RCCa.