We performed a prospective, controlled trial of recombinant leukocyte A interferon (IFN-alpha 2A) with or without aspirin (ASA) in 176 patients with assessable advanced renal cell cancer in light of a 34% response rate (10 of 29 patients) from the two-agent regimen in an earlier nonrandomized trial. This encouraging result was substantially higher than the 15% response rate typically achieved with IFN therapy alone. Eighty-seven patients received IFN-alpha 2A 20 x 10(6) U/m2 intramuscularly three times a week, and 89 received the same IFN therapy with ASA 600 mg orally four times each day. Each group was balanced as to relevant prognostic discriminants. Response rates were 8% for the group receiving ASA in addition to IFN, and 13% for the group receiving IFN alone (P = .30). The median times to progression were 1.9 months for the group receiving IFN with ASA and 2.7 months for the group receiving IFN alone (log-rank P = .36). The median survival durations were 8.8 months for the IFN and ASA group and 8.0 months for the IFN-only group (log-rank P = .60). These figures are also inferior to those typically reported from other studies. Our findings reemphasize the crucial role of randomized trials, admittedly cumbersome and time-consuming, to determine accurately the value of apparently promising therapies. Although some patients may derive benefit from IFN therapy, our findings raise disturbing questions regarding the potential IFN-alpha 2A according to the dose and schedule used in this trial to have any substantive impact on the ultimate outcome of disseminated renal cell cancer.