Point mutations of D835/I836 of the FLT3 gene have been reported in adult acute myeloid leukemia (AML), but not in pediatric AML or acute lymphoblastic leukemia (ALL). FLT3-D835/I836 mutations were found in 6 (5.4%) of 112 children with ALL older than 1 year and in 8 (16.0%) of 50 infants with ALL. Missense mutations were found in 11 patients, 3-base pair deletions in 2 patients, and a deletion/insertion in 1 patient. Remarkably, FLT3-D835/I836 mutations were found in 8 (18.2%) of 44 infants with ALL with MLL rearrangements and in 4 (21.5%) of 19 patients with hyperdiploid ALL, but they were not found in any patients older than 1 year who had TEL-AML1 (n = 11), E2APBX1 (n = 4), or BCR-ABL (n = 6) fusion genes. Although infant ALL patients with mutations had poorer prognoses than did those without mutations, pediatric ALL patients with mutations who were older than 1 year had good prognoses. We also found FLT3-D835 mutations in 2 of 11 leukemic cell lines with MLL rearrangements. FLT3 was highly phosphorylated in these cell lines with FLT3-D835 mutations, leading to constitutive activation of downstream targets such as signal transducer and activator of transcription 5 (STAT5) without FLT3 ligand stimulation. These results suggested that FLT3-D835/I836 mutations are one of the second genetic events in infant ALL with MLL rearrangements or pediatric ALL with hyperdiploidy.