Thiazolidinediones, a class of drugs for the treatment of type-2 diabetes, are synthetic ligands for peroxisome proliferator-activated receptor-gamma. They have been demonstrated to possess cardioprotective effects in humans and anti-atherogenic properties in animal models. However, the question remains whether a peroxisome proliferator-activated receptor-gamma ligand can reverse the development of atherosclerosis. In this study, we tested the effects of pioglitazone on the development of established atherosclerosis in low-density lipoprotein receptor-null mice. We observed that atherosclerosis in low-density lipoprotein receptor-null mice progressed when mice were fed a high-fat diet. Pioglitazone treatment of atherogenic mice prevented this progression of atherosclerosis from its middle stages of disease, but was not able to reverse it. Withdrawal of the high-fat diet from mice with advanced atherosclerosis did not result in a reduction in lesion sizes. Pioglitazone treatment also had no effect on advanced atherosclerosis. Levels of high density lipoprotein cholesterol correlated inversely with lesion development when pioglitazone was given during lesion progression. However, pioglitazone had no effect on circulating high density lipoprotein levels in mice in which treatment was initiated following 14 weeks on the high-fat diet. These findings have implications for the analysis of therapeutic agents in murine models of atherosclerosis and the use of pioglitazone in patients with established atherosclerosis.