Leukemias diagnosed in the first 12 months of life are characterized by an equal distribution of lymphoid and myeloid subtypes and account for 2.5% to 5% of acute lymphoblastic leukemias (ALLs) and 6% to 14% of acute myeloid leukemias (AMLs) of childhood. 1, 2 In contrast to an excess of boys among older children with leukemia, there is a slight female predominance among infants with this disease. 3-5 Infant leukemias display unique biological and clinical features that have provided important insights into the mechanisms governing normal and aberrant hemopoiesis in the fetus and young children, as well as reasons for the increased rates of treatment failure in infants as compared with older children. This review summarizes recent progress in understanding the biology of infant leukemias and the prospects for better treatment.