One hundred and twenty patients suffering from an AIDS-related Kaposi's sarcoma treated by 18 million units of recombinant alpha-2A-interferon daily were followed prospectively for a period of between one and six years. An overall complete response was observed in 35% of these patients; the figure was significantly higher in those who did not have a visceral localization or opportunistic infections. Total lymphocyte count, CD4 lymphocyte count, and CD4/CD8 ratio were significantly higher, and beta-2-microglobuline significantly lower, in the responders than in the non-responders. A multivariate analysis showed that localization of KS and CD4 count had independent predictive value, with an odds ratio of 35 for patients who had more than 300 CD4 cells at the onset of treatment versus those with less than 150. Patients whose initially negative p24 antigenemia remained negative during treatment had the highest frequency of complete response. Among patients with initially positive p24 antigenemia, those whose percentage decrease in antigenemia levels was greatest had a higher frequency of complete response. The cumulative probability of survival in responders was 62% at four years. These results demonstrate an anti-tumoral and anti-viral effect and prolonged survival in a group of patients whose initial immune parameters were relatively well preserved. However, these results do not permit us to conclude whether these well-responding patients were treated at the onset of illness, or whether their illness was naturally less evolutive.