OBJECTIVE The prognostic significance of CD34 antigen expression in acute lymphoblastic leukemias (ALL), especially in adult patients, is still not well established. In the present report we analyzed a series of biological and clinical findings from 128 ALL patients in order to evaluate the possible clinical significance of this marker. METHODS The clinical and biological significance of CD34 expression, an early marker of hemopoietic cells, was analyzed by flow cytometry in a series of 128 patients affected by ALL, including 78 adults and 50 children under 15 years old. RESULTS Overall, 68.7% of patients showed significant ( > 10%) CD34 expression. There was no difference between CD34+ and CD34- ALL with respect to age, sex, FAB morphology, hepatosplenomegaly, Plt count, Hb level, DNA index, P-170 expression. CD34+ ALL displayed a significantly lower frequency of extramedullary involvement, a lower LDH level and lower WBC count, lower proliferative activity (as evaluated by the Ki67 monoclonal antibody) than CD34- ALL. CD34 expression was also associated with early phenotypes in both B- and T-ALL, co-expression of myeloid antigens, and the presence of the Ph1 chromosome. Due to a different distribution of prognostic factors investigated, DFS and OS were both significantly better in CD34+ than in CD34- childhood ALL, whereas no statistical difference was found in adults. Multivariate analyses confirmed these data in children. CONCLUSIONS Expression of the CD34 antigen is a positive prognostic factor in childhood ALL. In adult ALL the presence of this marker on leukemic cell does not seem to influence the clinical outcome of these patients.