PURPOSE Reliable predictors of HIV disease progression are scarce in developing countries, where most HIV infections occur. We describe early virologic and immunologic events among men who have sex with men in Rio de Janeiro, Brazil. METHODS Seroconverters from 2 high-risk cohorts were followed for up to 36 months with periodic laboratory evaluations, plasma viral load, and CD4 count assessments. Viral load and CD4 count mean trajectories were computed. For the modeled viral loads, mean and median values were 24,480 (4.36 log10) and 19,720 (4.29 log10) copies/mL (range 14,880-58,090), respectively. Median CD4 count was 373 cells/microL (range 260-508). Overall variation on viral loads ranged from 4.3 to 5.2 log10 copies/mL with a visible increase in the viral load starting at approximately 600 days (n = 12) after estimated time of seroconversion. The initial period of HIV infection was characterized by an increase in CD4 count (n = 29) followed by a steep decline starting at approximately 200 days (508 cells, 95% CI: 425 to 569). A gradual decrease was observed in the median CD4 count thereafter, reaching 281 (95% CI: 100 to 466) at 1000 days after the estimated date of seroconversion. CONCLUSIONS Although viral load dynamics resembled those observed in developed countries, CD4 counts seem to decline at a faster rate than in the Multicenter AIDS Cohort Study (MACS) cohort. Clinical and survival data are needed to assess the impact of interventions, such as antiretroviral therapy, on the clinical course of HIV infection in Brazil.