The oncogene EVI1 has been implicated in the etiology of AML and MDS. Although AML cells are characterized by accelerated proliferation and differentiation arrest, MDS cells hyperproliferate when immature but fail to differentiate later and die instead. In agreement with its roles in AML and in immature MDS cells, EVI1 was found to stimulate cell proliferation and inhibit differentiation in several experimental systems. In contrast, the variant protein MDS1/EVI1 caused the opposite effect in some of these assays. In the present study, we expressed EVI1 and MDS1/EVI1 in a tetracycline-regulable manner in the human myeloid cell line U937. Induction of either of these proteins caused cells to accumulate in the G(0)/G(1)-phase of the cell cycle and moderately increased the rate of spontaneous apoptosis. However, when EVI1- or MDS1/EVI1-expressing cells were induced to differentiate, they massively succumbed to apoptosis, as reflected by the accumulation of phosphatidylserine in the outer leaflet of the plasma membrane and increased rates of DNA fragmentation. In summary, these data show that inducible expression of EVI1 in U937 cells causes phenotypes that may be relevant for its role in MDS and provides a basis for further investigation of its contribution to this fatal disease.