In spite of the high incidence of AIDS in Brazil, few studies have tried to evaluate the prognosis of asymptomatic HIV seropositive Brazilian patients. A hospital outpatient facility-based cohort of HIV seropositive asymptomatic subjects was followed to determine their probability of remaining AIDS-free at 2 and 4 years of follow-up, as well as the one-year estimated cumulative probability of survival for the AIDS incident cases. The cohort was made up of all asymptomatic HIV seropositive subjects referred to the Immunology Branch of a large university hospital in São Paulo, Brazil, between 1985 and June 1997. The cumulative probability of remaining free from AIDS was 79% (+/- 3.7% SE) at 2 years, and 64.4% (+/- 5.1% SE) at 4 years after first known positive anti-HIV serology. Women had a marginally significant better probability of remaining AIDS-free after both 2 and 4 years of known seropositivity, as compared with men. There were no significant differences in the prognosis of the infection by age; the only single parameter associated with better prognosis was an initial CD4+ count > or =350/microl. The probability of survival one year after the diagnosis of AIDS was 78%, and the 50% estimated probability of survival was 19 months. Older patients (aged > or =35 years) had a better prognosis, as suggested by their longer survival estimates (P = 0.06). The probability of survival with AIDS observed in this study was higher than in the few previously published estimates for Brazil. However, since the time frame was so wide, it may not be entirely comparable with earlier studies. Some likely explanations for this possibly better prognosis could include more efficient prophylaxis for opportunistic diseases, as well as an increase in the availability of anti-retroviral drugs. The 8% incidence of AIDS at 2 years observed in this study for those individuals whose initial CD4+ count was > or =350/ml was close to that found in a large international epidemiological study of seroconverters.