Forty-eight Japanese infants with acute lymphoid leukemia (ALL) (n = 24) and acute nonlymphoid leukemia (ANLL) (n = 24) were analyzed on the basis of clinical and laboratory data. Morphologically, 20 of the 24 infants with ALL were of the FAB L1 type, and 20 of the 24 infants with ANLL were of the M4 or M5 type. Markedly enlarged liver and spleen, and hyperleukocytosis (more than 50,000/microliters) were seen in 9, 12, and 14 infants with ALL and 10, 11, and 10 infants with ANLL, respectively. Initial CNS leukemia was evident in 2 infants. Chromosome studies of the leukemic cells showed abnormal karyotypes in 9 of the 21 infants with ALL and 19 of the 22 infants with ANLL, consisting mainly of translocation 11, 12, and inversion 16. By surface marker analysis, only 7 of the 22 infants with ALL (32%) were diagnosed as having common ALL (HLA-DR+, CD19+, CD10+). Of the 15 infants with ANLL, 12 and 5 infants also showed reactivity to HLA-DR and CD19, respectively. All of the 5 ANLL infants with lymphoid markers showed different leukemic cell features at the time of relapse. Twelve of the 19 infants with ALL (63%) who achieved a complete remission relapsed within the first 2 years; 8 of the 21 with ANLL (38%) relapsed within the first year. Analysis of event-free survival shows that the ALL infants with hyperleukocytosis have a poorer prognosis than those without hyperleukocytosis (p less than 0.05). Infant leukemia originates in a multipotent cell with lymphoid and myeloid features, and intensive multiagent chemotherapy is necessary for the treatment of infants with acute leukemia.