Background: PPAR. is a member of the nuclear hormone receptor superfamily. It has been considered as a mediator regulating metabolism, anti-inflammation, and pro-proliferation in the Vascular Smooth Muscle Cells (VSMCs). Thiazolidinediones (TZDs), synthetic ligands of PPAR gamma, have anti-proliferative and pro-apoptotic effects on VSMCs, which prevent the formation and progression of atherosclerosis and restenosis following percutaneous coronary intervention (PCI). However, the underlying mechanism remains elusive. This present study therefore aimed to investigate the signaling pathway by which pioglitazone, one of TZDs, inhibits proliferation and induces apoptosis of VSMCs. Methods: The effects of pioglitazone on VSMC proliferation and apoptosis were studied. Cell proliferation was determined using BrdU incorporation assay. Cell apoptosis was monitored with Hoechst and Annexin V staining. The expression of caspases and cyclins was determined using real-time PCR and Western blot. Results: Pioglitazone treatment and PPAR gamma overexpression inhibited proliferation and induced apoptosis of VSMCs, whereas blocking by antagonist or silencing by siRNA of PPAR gamma significantly attenuated pioglitazone's effect. Furthermore, pioglitazone treatment or PPAR gamma overexpression increased caspase 3 and caspase 9 expression, and decreased the expression of cyclin B1 and cyclin D1 in VSMCs. Conclusions: Pioglitazone inhibits VSMCs proliferation and promotes apoptosis of VSMCs through a PPAR gamma signaling pathway. Up-regulation of caspase 3 and down-regulation of cyclins mediates pioglitazone's anti-proliferative and pro-apoptotic effects. Our results imply that pioglitazone prevents the VSMCs proliferation via modulation of caspase and cyclin signaling pathways in a PPAR gamma-dependent manner.