The evolution of T-lymphocyte subsets during infancy in perinatally HIV-infected African babies has not been previously described. In a hospital-based cohort study, T-lymphocyte subset changes were investigated in 72 South African black children born to HIV seropositive mothers. Sixteen (22.2%; children were classified as infected and 56 (77.8%) as uninfected by 18 months of age. Four (25%) of the infected infants died before the age of 9 months from HIV-related disease. The CD4 and CD8 T-lymphocyte subsets, expressed in absolute numbers, as percentages, percentiles or as ratios, were clear indicators of HIV infection at all ages between 3 and 15 months. The most marked changes were a decreased percentage of CD4 cells and an increase in percentage of CD8 cells in the infected group. In the 4 infected infants who died, CD8 count and CD4:CD8 ratio clearly predicted poor clinical outcome at 3 months. Taken together, both CD4:CD8 ratio and CD4 percentage are reliable markers of HIV infection in an African paediatric population; however, a raised CD8 lymphocyte count rather than a CD4 count is a more specific prognostic marker of disease progression in HIV infected children.