Patients with refractory advanced or metastatic urothelial carcinoma derive only minor benefit from chemotherapy. Based on evidence that urothelial carcinoma may be associated with impaired immunological reactivity, we conducted a phase II trial of interleukin-2 (IL-2), a biologic response modifier, to assess its efficacy and toxicity in treating refractory advanced or metastatic urothelial carcinoma. Seventeen patients with urothelial carcinoma who had undergone no more than 1 cisplatin-containing chemotherapy regimen were treated with IL-2 at a dose of 3 x 10(6) IU/m(2)/day by continuous intravenous infusion for 4 consecutive days each week for 4 weeks. Cycles were to be repeated every 6 weeks until disease progression. Toxic effects could be assessed in all 17 patients and response in 13. The most common grade III and IV toxic effects included hypotension (13/17); anemia (6/17); thrombocytopenia (4/17); granulocytopenia (3/17); and, in 1 patient each, cardiac ischemia, bowel perforation, and an increase in creatinine level. One sudden death was assumed to be treatment related. Although we found no objective antitumor activity for IL-2, median patient survival was 10.5 months (95% confidence interval, 5.8 to 17.1 months), with a 15.9-month median survival for 3 patients with poor performance status and visceral metastases. Further clinical investigation of the biological effect of IL-2 in urothelial carcinoma may be warranted.