A randomized double-blind controlled trial was conducted to determine the efficacy of passive immunotherapy in the treatment of symptomatic human immunodeficiency virus (HIV) infection. This trial included 86 symptomatic patients randomized to receive plasma rich in anti-HIV-1 antibody or standard seronegative plasma. Each patient in both groups received a 300-ml infusion every 14 days over a 1-year period, and every 28 days thereafter, in addition to zidovudine and other conventional prophylactic treatments. Plasma donors were selected among symptomless seropositive individuals with a CD4 lymphocyte count > or = 400 x 10(6) cells per liter, a negative p24 antigen assay, and a high concentration of anti-p24 antibody. The plasmas were heat-inactivated before infusion. During the study period (day 28-day 365) scheduled by the protocol, clinical benefit from passive immunotherapy was observed in delaying the appearance of the first AIDS-defining event (P < 0.009) and reducing the cumulative incidence of such events, which was estimated 3-fold higher in the control group compared to the treatment group. Seven deaths occurred in the treatment group vs. 11 in the control group (P = 0.27). A total of 47 patients died or exhibited new AIDS-defining events, 18 in the treatment group and 29 in the control group (P = 0.009). No clinical benefit was observed after the 1-year period with infusions performed every 4 weeks. These results indicate a favorable effect of passive immunotherapy on the evolution of advanced AIDS.