BACKGROUND Sunitinib malate is an oral, multitargeted tyrosine kinase inhibitor that has demonstrated superior efficacy over interferon (IFN)-alpha in a phase III trial in first-line, metastatic renal cell carcinoma (RCC). Herein, we report the results of a phase I dose-finding study of sunitinib in combination with IFN-alpha as first-line treatment in patients with metastatic RCC. METHODS Treatment-naive patients with clear-cell metastatic RCC received sunitinib at a starting dose of 50 mg or 37.5 mg orally once daily in 6-week cycles (schedule 4/2) plus IFN-alpha at a starting dose of 3 MU subcutaneously 3 times a week, with weekly intrapatient dose escalation to a maximum of 9 MU as tolerated. Patients who did not tolerate either drug received lower doses of either or had dose interruptions. RESULTS Twenty-five patients were enrolled; their median age was 64 years (range, 45-77 years). All patients experienced grade 3/4 treatment-emergent adverse events; the most common were neutropenia, fatigue, and thrombocytopenia. After a median of 4 cycles (range, 1-9 cycles), 3 patients (12%) had a partial response, and 20 (80%) had stable disease. CONCLUSIONS Although reduced starting doses were tolerated (37.5 mg for sunitinib and 3 MU for IFN-alpha), even these lower doses might not be well tolerated for long-term treatment of patients with metastatic RCC. Based on historical data, sunitinib on schedule 4/2 appears to be more effective as single-agent therapy. Further study of sunitinib plus IFN-alpha on this schedule is not being pursued in RCC.