The intravenous immunotherapy with interleukin 2 (IL-2) represents one of the most active therapies of metastatic renal cell carcinoma (RCC). Recently, it has been demonstrated that IL-2 given subcutaneously in association with interferon alpha (IFN) may determine a response rate in RCC comparable to that obtained with an intravenous route of administration, but with a lower toxicity. Moreover, our previous data have suggested that IFN is not essential for IL-2 efficacy. On the basis of these data, we have designed a protocol of immunotherapy with IL-2 alone given subcutaneously in the treatment of metastatic RCC. The study included 48 consecutive evaluable patients. IL-2 was given at a daily dose of 6 million IU for 5 days/week for 6 consecutive weeks, corresponding to one IL-2 cycle. The overall response rate was 14/48 (29%; CR:1; PR: 13). Response rate was significantly higher in nephrectomized than in nonnephrectomized patients, and in patients with a good compared to those with a low performance status. Patients with an interval between the diagnosis of primary renal tumor and of its metastases longer than 1 year did better than those with a lower interval, as did patients with a single metastasis compared to those with multiple metastases, while no significant difference was seen in relation to sex, age and previous IFN therapy. As far as dominant metastasis sites are concerned, patients with liver metastases showed a response rate significantly lower than that seen in patients with metastases in sites other than liver. Toxicity was low in all patients. This study shows that the subcutaneous immunotherapy with IL-2 alone is a well tolerated and effective therapy of metastatic RCC. The evidence of a low PS, disseminated tumor and liver metastases represents the most important negative prognostic factor for the response to therapy.