Immunophenotyping of peripheral blood lymphocyte subpopulations is essential for the diagnosis and follow-up of children with immunodeficiencies and other immune disorders. The relative size and absolute number distributions (median and 5-95%) of lymphocyte subsets, including cord blood (Coulter, EPICS-XL) were examined by flow cytometry in 190 healthy subjects from birth to 18 years of age with a view to obtaining normal reference values for Turkish children of the following age groups: cord blood (n:29), birth to 1 year (n:41), 1 to 2 years (n:30), 2 to 6 years (n:30), 6 to 10 years (n:30), and 10 to 18 years (n:30). The relative size of CD2+, CD3+CD16-56-, CD3+CD8+ T lymphocytes increased while the relative size and absolute counts of those together with CD3+CD4+ and CD19+, CD20+ B lymphocytes decreased with age. The percentage of CD3-CD16+56+ NK cells increased from 0-1 year to 10-18 years; however, absolute count of CD3-CD16+56+ NK cells remained stable and unchanged in all age groups. The relative size and absolute count of activation markers (CD3+CD25+ and HLADR+) decreased from 0-1 year through 10-18 years age group. This study has once more demonstrated that both the percentage and the absolute number of lymphocyte subsets in cord blood and peripheral blood of healthy infants and children changed with age. Therefore, comparison of results to those of age-matched healthy controls is of utmost importance in the reliable and accurate evaluation of lymphocyte subsets reflecting cellular immunity in children.