The phenotypes of malignant cells from 350 untreated patients with acute lymphoblastic leukemia (ALL) were determined at diagnosis with the use of a panel of monoclonal antibodies to leukocyte antigens. According to the phenotypes seen, the cases were divided into five groups, pre-B ALL, B-ALL, T-ALL, MO-ALL, and undifferentiated ALL. Each group was subdivided, resulting in 11 defined immunologic subtypes. Correlations between clinical and laboratory features were investigated at presentation. ALL of early-B phenotype associated with elevated cell counts occurred more often in female and infant patients than in male patients. Involvement of the central nervous system was frequent in B-ALL, which occurred mostly in male patients. A male prevalence was also seen in ALL of T-lineage in which significant differences regarding clinical characteristics and leukocyte counts appeared among the four subtypes. The clinical relevance of phenotypic subcategorization is supported by our observations.