The survival of 35 patients with AIDS or advanced HIV infection on treatment with zidovudine was related to the viral sensitivity to the drug and to the CD4+ cell count. 14 patients died, the survivors were followed up for an average of 804 days. In a univariate Cox model, survival was strongly related to log IC90 (p = 0.0003) and to the CD4+ count (p = 0.0002). In a bivariate model, log IC90 and the CD4+ count contributed to the prediction of survival (p = 0.12 and 0.06, respectively). Large studies of combination or alternation therapy with several anti-HIV drugs should be given high priority.