In order to assess the age-related changes in CD10 and CD19 fluorescence intensity (FI) the present study analyzed by flow cytometry 56 sternal biopsies from 'normal' infants, children and adults undergoing cardiac surgery. The CD10(+weak) subset was predominant in all age groups, representing approximately 50% of the bone marrow (BM) lymphoid cells in children younger than 4 years. Both CD10+ subsets significantly decreased with age but their ratio did not differ significantly. Moreover, the intensity of CD10 and CD19 fluorescence in the strong and weak subsets did not vary with age. The CD19 intensity was significantly higher in CD10(+weak) than in CD10(+strong) cells. In addition, we classified as CD10(+weak) or CD10(+strong) the leukemic cells from BM aspirates of 117 patients with common acute lymphoblastic leukemia (cALL) (78 children and 39 adults). A higher frequency of cases expressing the CD19+ CD10(+strong) phenotype was observed both in children and adults. Children of the CD10(+weak) group tended to be older than those of the CD10(+strong) group (median = 7 vs. 4 years, P = 0.07), and presented a significantly higher frequency of splenomegaly (93.7 vs. 55%, P = 0.04), which was massive in about 60% of these cases. Among adults, a significantly higher frequency of cases expressing the CD10(+weak) phenotype was observed in females. No other clinical or biological difference was detected between the two groups either for children or adults. Concerning the treatment outcome, we did not observe significant differences in complete remission rate (CRR) or in disease free survival (DFS) among the 32 children and 28 adults analyzed. Finally, we compared the CD10 and CD19 intensity in normal and leukemic BM. Overexpression of either or both antigens in leukemic cells was observed in 42.4% of the cALL cases. In these cases, using cut off values of 110 afu for the CD10 FI and of 100 afu for the CD19 FI, the detection of leukemic cells was possible at levels of 0.2% based on CD10 analysis, of 0.6% based on CD19, and 0.02% when both antigens were overexpressed. In conclusion, we demonstrated that the heterogeneity of CD10 and CD19 fluorescence intensity is of no clinical relevance in cALL, although its study may be helpful for the diagnosis and the detection of minimal residual disease.