Based on 392 infected children enrolled in two European prospective studies of infants born to HIV-infected women, with similar standard protocols, HIV disease progression in the first 6 years of life is described, using the 1994 CDC paediatric HIV classification. Most children had developed minor (A) or moderately severe (B) illness in the first 4 years of life, although usually it was transient in nature. Progression to U.S. Centers for Disease Control and Prevention (CDC) group C disease or HIV-related death is an estimated 20% (95% confidence interval 16-24%) during the first year of life, and 4.7% (3.3-6.5%) per year thereafter, giving a cumulative incidence of 36% (30-43%) by 6 years. The mortality rate at 6 years is 26% (20-32%). Two thirds of the children alive at 6 years had only minor symptoms, and one third had a CD4+ cell distribution of > 25% despite previous clinical manifestations and a transient period of moderate immune deficiency. Differences in zidovudine monotherapy between the two cohorts were not associated with the mortality rate. However, the risk of severe bacterial infections was lower in the French cohort, in which the use of antibacterial prophylaxis was more common. The early, severe form of HIV disease affects approximately 20% of infants, and after 6 years 75% of infected children are still alive. This has important implications for health-care planning.