To assess the safety and efficacy of combined therapy with interferon-alpha (INF-alpha) and active vitamin D(3) for metastatic renal cell carcinoma (RCC). Sixteen patients with metastatic RCC were enrolled in this prospective study. All received oral alfacalcidol (1 microg once daily) and INF-alpha (Sumiferon; 3 million units, three times a week). The primary endpoint was the response rate (defined as complete + partial remission). Secondary endpoints were cancer-specific survival and toxicity. The median follow-up period was 17 months (range: 5-49 months). The median age of the patients was 68 years (range: 41-73 years). The sites of metastases were: lung in 13 patients, bone in one, lung and bone in one, and lung, bone, and lymph nodes in one. Four patients (25%) had a partial response (PR), 10 patients (62.5%) showed no change (NC), and two patients (12.5%) had progressive disease (PD). The median cancer-specific survival time was 45 months. One patient had to discontinue vitamin D(3) because of hypercalcemia. Kaplan-Meier survival analysis revealed that metastasis at the time of initial diagnosis and older than average age were significant predictors of poor survival (P < 0.05). Combined treatment with INF-alpha and active vitamin D(3) has shown to be safe and effective for metastatic RCC patients.