Using cell surface markers and a panel of monoclonal antibodies, 515 cases of acute lymphoblastic leukemia (ALL) were immunophenotyped. T cell type ALL (T-ALL), non-T cell type ALL (Non-T-ALL) including common ALL (C-ALL), Null-ALL and B cell type ALL (B-ALL) were found. These major subtypes of ALL were further divided according to their phenotypes in detail. It was noticed that the phenotypes of these subtypes of ALL reflected basically the phenotypes of normal T or B cells at various differentiation stages or certain population of lymphocytes. The diagnosis of cell lineage was more precise when based on immunophenotyping than morphological description. The combination of morphological and immunological classification can improve the diagnosis of acute leukemias. In addition, it was observed that the immunophenotyping was relevant to clinicopathologic features, responses to therapy and prognosis of ALL patients. The incidences of major subtypes of ALL, the age distribution of ALL subsets and male sex bias with T-ALL in Chinese are discussed.