Assessment of the percentage and absolute number of T cells as well as of their main subpopulations is presently a routine procedure for the diagnosis and follow-up of a wide array of pediatric immunologic disorders. For several clinical applications (severe immunodeficiencies or leukaemias) the diagnostic usefulness of their enumeration does not require close comparison with age normal values, while in other circumstances such as follow-up of immunomodulating or immunosuppressive treatments or detection of minor immune defects, the expected changes of T cell subsets are more subtile and they are likely to be detected only by comparison with well-defined age normal values. In the present study CD3, CD4 and CD8 positive cells were enumerated in a group of 410 healthy children of age ranging from 30 days to 9 years. No significant changes in percentage or absolute number were observed during infancy and childhood. Furthermore the sum of CD4 and CD8 positive cells was close to the percentage of CD3 positive cells, suggesting a phenotype maturity of T cells from infancy.