There is a paucity of information in the literature concerning the age‐related changes of the lymphocyte subsets in bone marrow (BM), and the available reports disagree about the characteristics of the population studied and the methods for obtaining, handling, and analyzing the samples. The purpose of the present study was to determine the distribution of lymphoid subsets in the BM from infants, children, and adults by analyzing fragments of sternum obtained during cardiovascular surgery. The samples were studied by flow cytometry employing the whole blood lysis method and excluding from the analysis the contamination of the lymphoid window by erythroid precursors. We observed that in the first 4 years of life the B subset represented more than 65% of all cells in the lymphoid window, most of them (80%) exhibiting the immature phenotype CD19+CD10+. Conversely, the T subset was composed of mature CD4+ or CD8+ cells, with the CD4/CD8 ratio being less than 1 in all age groups. With age there was a progressive decrease in the percentage of B cells and an increase of T cells, reaching similar proportions in the BM from adults (33.6% and 34.8%, respectively). Furthermore, the percentage of CD10+ cells in the B subset decreased independently, whereas the CD20 expression increased. The percentage of NK cells did not change with age. Cytometry (Comm. Clin. Cytometry) 34:22–29, 1998.