Fifty adult patients with acute lymphoblastic leukemia (ALL) were prospectively studied to determine the clinical and hematological relevance of surface immunophenotypes. Before treatment, blast cells were assayed for reactivity to monoclonal antibodies to B-cell, T-cell, and myeloid (My) antigens. My antigens (CD13, CD33, and VIM2, singly or in combination) were demonstrated in 16 cases (32%) along with lymphoid specificities. Bone marrow and peripheral blood stains were classified according to French-American-British (FAB) Cooperative Group criteria and evaluated for myelodysplastic changes and azurophilic granules. Mean age of My+ patients was significantly higher. Furthermore, a greater number of My+ cases showed azurophilic cytoplasmic granules and acid ANAE positivity. FAB subtypes and myelodysplastic features did not significantly differ in the two groups analyzed, but patients with myelodysplastic abnormalities represented a significantly older age group. Response to treatment was comparable in My+ and My- cases, in terms of either complete remission rate or median survival duration.