BACKGROUND Interleukin-2 is the most promising antitumor agent for advanced renal cell carcinoma, but systemic immunotherapy with interleukin-2 might be limited because of inadequate efficacy and severe adverse effects. In this study, we treated 7 patients with lung metastases from renal cell carcinoma with topical application of interleukin-2 by inhalation. METHODS Patients received 100,000 IU of interleukin-2 by inhalation 4 times a day and 9,000,000 IU of interferon-alfa-2a subcutaneously for 5 consecutive days per week. They also received, by oral administration, 800 mg of cimetidine and 50 mg of indomethacin per day. After informed consent was obtained, the treatment started and the absence of any intolerable adverse effects was confirmed in a hospital. Then the treatment continued in an outpatient clinic for at least 3 months. RESULTS Of 6 assessable patients, 5 responded to this treatment; 2 patients developed a partial response (33%) and 3 remained stable (67%). Disease progressed in the remaining patient. Therapy was discontinued in 1 patient because of his poor general condition. No severe adverse effects were observed, but pulmonary fibrosis probably associated with this treatment occurred in 1 patient. CONCLUSIONS Although more cases and further evaluation are necessary to assess the significance and the safety of the inhalation of interleukin-2, this treatment is anticipated to be an option for selected patients with lung metastases from renal cell carcinoma.