Acute lymphoblastic leukemia (ALL) is the most common childhood malignancy and, in most cases, is of pro- or pre-B cell origin (B-ALL). The receptor tyrosine kinase KIT is expressed by hematopoietic stem and precursor cells. Gain-of-function mutations of KIT cause systemic mastocytosis, which is characterized by abnormal accumulations of mast cells. We previously reported a mouse model of mastocytosis based on conditional expression of a constitutively active Kit protein. Half of these animals developed leukemic disease of B-lineage origin. Herein, we report that this condition bears striking similarities to human B-ALL. The immuno-phenotype of the leukemic cells was compatible with a pro-B- cell origin, as was the finding of immunoglobulin heavy-chain gene rearrangements in all cases, whereas light-chain loci were mostly not rearranged. Leukemogenesis was independent of pre-B-cell receptor expression. Primary leukemic cells and permanent cell lines derived from these were serially transplantable and rapidly killed the recipients. In a few animals, the leukemia was of T-cell origin with abnormal CD4/8 double-positive T-cell precursors dominating in the circulation. In summary, we report a novel ALL mouse model that may prove useful for in vivo drug testing and identification of novel oncogenic mutations and principles.