Seventy-five adult patients with newly diagnosed acute lymphoblastic leukemia (ALL) were analyzed for CD34 expression on leukemic cells. CD34 was significantly associated with B-cell lineage ALL (p = 0.0002). In B-lineage ALL, CD34 positivity was significantly associated with expressions of CD9 (p = 0.001), CD19 (p = 0.00001) and CD22 (p = 0.002). CD34 was more expressed in B-ALLs with higher WBC cell count (p = 0.04), and higher percentage of peripheral blood leukemic cells (p = 0.005), total or partial monosomy of chromosome 7 (p = 0.0001) or Ph+ chromosome (p = 0.01); and less expressed in cases with hyperdiploidy (> or = 50 chromosomes) (p = 0.03). CD34 was more expressed in poor risk B-ALLs patients, defined according to Hoelzer criteria (p = 0.01). In T-lineage ALL, CD34 positivity was inversely correlated with the expression of CD10 (p = 0.05). After intensive induction therapy, 58 of 73 evaluable patients (79%) achieved a complete remission (CR). CD34 positivity was correlated with the persistence of blast cells in day 15 bone marrow aspirates (p = 0.001) and after one course of induction chemotherapy (p = 0.01). With a median follow-up of 11 months, no statistical differences were seen in leukemia-free survival and overall survival between CD34 positive and negative cases, even when stratifying by immunophenotype. We conclude that CD34 expression is associated with features of poor prognosis in adult ALL. Its study might therefore become useful in the design of future prognostic models.