A longitudinal study of lymphocyte subsets during infancy was evaluated by using the flow cytometric immunophenotyping method. Two hundred and thirteen blood samples were obtained from 92 healthy, full-term infants of the following ages: 1-7 days old (n = 43), 3 months old (n = 55), 6 months old (n = 57) and 11 months old (n = 58). The absolute numbers of CD3+ and CD3+/CD4+ T lymphocytes increased from birth to 3 months of age, and remained stable thereafter. The absolute number of CD3+/CD8+ T lymphocytes increased from birth to 11 months of age. The absolute number of CD19+ B lymphocytes and NK cells increased rapidly (3 months) after birth and continued to increase throughout the study period. However, the changes in the relative counts of lymphocyte subsets did not always correspond with the changes in their absolute numbers. These results demonstrate the age-related changes in lymphocyte subpopulations and provide reference ranges for lymphocyte subsets during infancy.