The efficacy and toxicity of interferon-alpha2a (9MU/d) and bleomycin (15 mg every 2 weeks), each combined with zidovudine (2x250 mg/d), was compared in a randomized study in 26 men with progressing AIDS-related Kaposi's sarcoma (KS). The median CD4 count was 113/microl. Complete or partial response was achieved in one (8%) of 12 evaluable patients on interferon and in 2 (20%) of 10 patients on bleomycin (P = 0.43) during 4.7 and 5.3 months of treatment, respectively. The tolerability was comparable. During extended follow up, survival time was 24 and 13 months in the interferon and bleomycin arm, respectively. In a multivariate Cox regression analysis, CD4 lymphocytes <200/microl (relative risk 3.74; 95% CI: 1.30-10.8) and randomization to interferon (relative risk 0.37; 95% CI: 0.15-0.90) were significantly predictive of mortality. New AIDS-related events occurred more frequently in patients who had received bleomycin. The antiviral activity of interferon-alpha or the chemotherapy-mediated increase in the risk for opportunistic infections may explain these differences.