The family of statins includes pharmacologic inhibitors of the 3-hydroxy-3-methylglutaryl-CoA reductase that are potent regulators of cholesterol biosynthesis. In addition to their cholesterol-lowering effects, statins inhibit cell proliferation and promote apoptosis of malignant cells in vitro, but their potential therapeutic roles in the treatment of malignancies remain to be defined. We examined the effects of statins on the growth and differentiation of acute myeloid leukemia (AML) cells. Atorvastatin and fluvastatin were found to be potent inducers of cell differentiation and apoptosis of the NB4 acute promyelocytic leukemia (APL) cell line. Such effects correlated with activation of the small G-proteins Rac1/Cdc42 and downstream engagement of the c-Jun NH(2)-terminal kinase kinase pathway, whose function was found to be essential for the generation of proapoptotic responses. Importantly, different statins were found to enhance all-trans-retinoic acid (ATRA)-dependent differentiation of APL blasts and reverse resistance to the antileukemic effects of ATRA. In addition, fluvastatin exhibited growth-inhibitory properties on primary bone marrow-derived leukemic progenitors from patients with AML and enhanced the suppressive effects of ATRA on leukemic progenitor colony formation. Altogether, these studies establish that statins exhibit potent antileukemic properties in vitro and raise the possibility that combinations of statins with ATRA may be an effective approach to overcome the development of ATRA resistance by the leukemic cells.