The study of cell surface antigens in acute lymphoblastic leukemia (ALL) has provided the basic information for classification of ALL into B cell or T cell lineage. Each immunologic type may present at an immature or more mature stage of maturation. This classification has prognostic significance since mature B cell and T cell ALL phenotypes have a worse prognosis, compared to common acute lymphoblastic leukemia-associated antigen (CALLA) positive leukemias that belong to an immature B cell lineage and have the best prognosis. By performing these immunologic studies together with those of molecular biology, it is now possible to establish the precise level of cell differentiation, the point at which the malignant transformation occurred. Further studies may allow correlation of these different maturation stages, thus providing insight into the biologic behavior of lymphoblastic leukemia.