The erythroleukemia developed by spi-1/PU.1-transgenic mice is a model of multistage oncogenic process. Isolation of tumor cells representing discrete stages of leukemic progression enables to dissect some of the critical events required for malignant transformation. To elucidate the molecular mechanisms of multistage leukemogenesis, we developed a microarray transcriptome analysis of non tumorigenic (HS1) and tumorigenic (HS2) proerythroblasts from spi-1-transgenic mice. The data show that transcriptional up-regulation of the sphingosine kinase gene (SPHK1) is a recurrent event associated to the tumorigenic phenotype of these transgenic proerythroblasts. SPHK1 is an enzyme of the metabolism of sphingolipids which are essential in several biological processes including cell proliferation and apoptosis. HS1 erythroleukemic cells engineered to overexpress the SPHK1 protein exhibited growth proliferative advantage, increased clonogenicity and resistance to apoptosis in reduced serum level by a mechanism involving activation of the ERK1/2 and PI3K/AKT pathways. In addition, SPHK1-overexpressing HS1 cells acquired tumorigenicity when engrafted in vivo. Finally, enforced expression of a dominant-negative mutant of SPHK1 in HS2 tumorigenic cells or treatment with a pharmacological inhibitor reduced both cell growth and apoptosis resistance. Altogether, these data suggest that overexpression of the sphingosine kinase may represent an oncogenic event during the multistep progression of an erythroleukemia