We examined the T-lymphocyte phenotypes of 67 human immunodeficiency virus (HIV)-infected children (P-1 or P-2) and 65 age-matched, healthy, control children stratified into four groups from < 1 to > or = 5 years of age to determine expression of antigens associated with cell activation/differentiation. Immunophenotyping was performed by laser flow cytometry using two-color immunofluorescent labeling. Although the control children showed a decline in total CD4 cell percent with age, the HIV-infected children in all age groups showed significantly decreased CD4 cell numbers compared with the age-matched controls. However, the slope of the CD4 cell decline with age was not significantly different in HIV-infected and control children. The CD4 cell decrease in infected children was reflected in both the CD45RA+ (naive) and CD45RA- (memory) CD4 cell subsets, although the CD45RA+ cells were decreased in greater proportion. Results assessing CD4 cells for expression of the L-selectin (Leu8) molecule were similar to those for CD45RA. The overall CD8 cell percentage was significantly increased in HIV-infected children compared with controls in all age groups. This was due primarily to increases in CD8 cells that were CD38+, CD57+, HLA-DR+, or CD45RA-. In a retrospective analysis of data from 23 P-0 children, we compared phenotype results from 5 children who were HIV+ with those 18 who were HIV-. Although the phenotypic changes seen in the 5 HIV+ children paralleled those described above for P-1 and P-2 subjects, there was no significant difference in the values for HIV+ compared with HIV P-0 children. Although the phenotypic alterations described did not appear to be diagnostic markers in P-0 children, they may serve as useful adjuncts for the evaluation of HIV-infected children.