Peripheral blood lymphocyte subset reference ranges were examined in a large group (N = 130) of healthy pediatric patients ranging in age from 1 month to 17 years. All samples were stained with monoclonal antibodies, processed with a whole blood lysis technique, and analyzed on a flow cytometer. Data analysis demonstrated statistically significant changes in most lymphocyte subsets at age 3 years. The relative and absolute numbers of total lymphocytes, CD2, CD4, and CD19 cells; absolute numbers of CD3 and CD8 cells; and CD4/CD8 ratios were high at birth, decreased during early childhood, and closely approximated adult reference values after age 3 years. The relative numbers of CD8 lymphocytes were low in early childhood and then rose to adult values after 3 years of age. The relative percentage of CD3 cells remained stable over all ages studied. Although "adult" lymphocyte subset reference ranges may be similar to those in children older than 3 years, age-adjusted reference ranges should be used for the early childhood period.