Clinical and laboratory characteristics at diagnosis, and long-term (up to 7 years) evolution, were analysed in 156 patients (66 adults and 90 children) with acute lymphoblastic leukemia (ALL), according to the expression of CD34. Eighty-three patients had CD34+ and 73 patients CD34- blasts at diagnosis. In adults, CD34+ ALL was more often of B-lineage (p = .003). White blood cell counts were higher both in children with CD34-B-lineage ALL (p = .02) and adults with CD34-T-lineage ALL (p - .05). Adult patients with CD34-T-lineage ALL were older (p = .04) and had more frequent splenic or liver enlargement (p = .02). Children with CD34+ ALL had a significantly (p = .04) longer event-free follow-up than CD34- children. Survival curves showed that after induction of remission, relapses were less frequent during the first 2 years for patients with CD34+ B-lineage ALL. After 2 years, however, event-free survival curves of CD34+ and CD34- patients merged. Thus it seems that CD34+ has no significant benefit for ALL patients.