To determine the efficacy of recombinant human leukocyte alpha-interferon (IFL-RA) in advanced hormone-refractory prostate cancer, the authors treated 40 patients with IFL-RA administered intramuscularly at a dose of 10 x 10(6) U/m2 three times weekly. Toxicity was substantial and necessitated at least a 50% dose reduction in all but five patients during the first 1-2 months of therapy. No responses were observed in patients with bone metastases, but complete and partial regression of nodal disease were observed in two patients with extraosseous disease (overall response rate, 5%; 95% confidence interval, 0.64-17.75%). The authors conclude that IFL-RA cannot be recommended at this dose and schedule in patients with advanced prostate cancer, but additional study of its use in patients with nodal disease may be warranted.