The antiproliferative effect of simvastatin on tumor cells has been speculated to be by intracellular signal inhibition through 3-hydroxy-3-methylglutaryl acetyl coenzyme A reductase. We examined the killing effect of simvastatin on imatinib-sensitive and resistant chronic myelogenous leukemia (CML) cells (three kinds of CML cell lines representative of each hematopoietic lineage: K562, KCL22, and LAMA84) and T315I and E255K site-directed mutant cells (Ba/F3). The in-vivo effect of simvastatin was determined in K562-xenografted nude mice. Cotreatment with imatinib and simvastatin in imatinib-resistant CML cells showed a synergistic killing effect in K562-R, KCL22-R, LAMA84-R, and E255K mutant cells, but only an additive effect in the T315I mutant cell, although a single treatment of simvastatin strongly inhibited T315I mutant cells. Mechanisms of killing were an induction of apoptosis and cell cycle arrest, through inhibition of tyrosine phosphorylation, and activated STAT5 and STAT3. Simvastatin suppressed the growth of K562-transplanted tumors, and cotreatment with imatinib was more effective in reducing tumor size. Simvastatin also killed primary CD34+ cells from patients with CML more efficiently, compared with CD34− CML cells. Our study shows a synergic effect of imatinib and simvastatin both in imatinib-sensitive and imatinib-resistant cells, but more effective synergism in resistant cells. On the basis of these findings, we suggest that a combination of simvastatin and imatinib may be a potential candidate for the treatment of imatinib-resistant CML.