In order to evaluate the biological features and clinical significance of myeloid antigen expression in acute lymphoblastic leukemia (ALL), immunophenotype analysis was performed on leukemic cells from 160 patients diagnosed as ALL by the French, American and British (FAB) criteria using a comprehensive panel of monoclonal antibodies to lymphoid and myeloid associated antigens. Expression of myeloid antigens was found in 32 cases (20%), including 11 out of 49 adults (22.4%) and 21 out of 111 children (18.9%). CD33 was positive in 18 patients (11.3% of the total cases), CD13 in 15 patients (9.4%), CD 11b in 12 patients (7.5%) and CD14 in 1 patient (0.6%). Nine patients expressed two or more myeloid antigens. There were no significant differences in clinical manifestations and hematological pictures between the ALL patients with myeloid antigen expression (My+ ALL) and those without (My- ALL). No consistent chromosomal abnormality was found in My+ ALL. Eighty percent of the childhood and 44.4% of the adult My+ ALL patients achieved complete remission, compared with 87% and 80%, respectively, for childhood and adult My- ALL, but the differences were not statistically significant. After a median follow-up of 2.1 years, there were also no statistically significant associations between myeloid antigen expression and shorter duration of remission or poorer survival rate for patients with both adult and childhood ALL.