Remarkable progress has been achieved in our understanding of the biology of acute lymphoblastic leukemias (ALL) occurring in childhood. Morphologic, immunologic, and cytogenetic studies of ALL have resulted in more accurate classification and risk assessment, permitting tailoring of therapy in this heterogeneous group of diseases. Studies of tumor cell biology have begun to pinpoint genes whose altered functions may be etiologically linked to cancer, and have provided insights into the biochemical processes involved in leukemogenesis.