The outcome of treatment of advanced renal cell carcinoma is disappointing. In interferon (IFN)-treated patients, the high incidence of adverse effects causes many patients to withdraw from treatment. This 12-week randomized study compared the incidence of toxicity associated with high-dose IFN monotherapy (15 x 10(6) U thrice weekly) and treatment with the combination of low-dose IFN (5 x 10(6) U thrice weekly) and 6 mg/m2 vinblastine (VBL) every 14 days in 100 consecutive patients. There was no significant difference in response rate between treatment arms (42% IFN vs. 34% IFN + VBL) or between subgroups (by tumor location). Combined treatment was associated with a significantly lower incidence of fever, fatigue, and weight loss but with a higher incidence of leukopenia. There was no significant difference in the incidence of other events. More patients treated with IFN monotherapy required bed rest, and overall treatment costs were 60% higher than for combined treatment. It is concluded that combined treatment with low-dose IFN and VBL, without loss of short-term efficacy, is better tolerated and less expensive than high-dose IFN monotherapy.