BACKGROUND Insulin resistance and hyperglycemia characterize type 2 diabetes mellitus. Type 2 diabetes mellitus is usually accompanied by concomitant disorders, such as dyslipidemia, hypertension and atherosclerosis. Thiazolidinediones are antidiabetic drugs that increase insulin sensitivity by activating the peroxisome proliferator-activated receptor gamma. There is evidence that thiazolidinediones exert a number of pleiotropic effects that may play an important role in the treatment of type 2 diabetes mellitus. OBJECTIVE The purpose of the present article was to review the 'pleiotropic' effects of thiazolidinediones (i.e., their effects beyond glucose lowering). METHODS The study involved searching PubMed up to February 2008 using relevant keywords. CONCLUSIONS Thiazolidinediones favorably alter fat distribution and improve cardiovascular risk factors, such as blood pressure, inflammation markers and uric acid and they may also delay the progression of atherosclerosis. The effects on the lipid profile differ between the two thiazolidinediones studied with pioglitazone having more positive effects compared with rosiglitazone. Furthermore, thiazolidinediones improve diabetic complications, such as diabetic nephropathy and non-alcoholic fatty liver disease. Thiazolidinediones may also play a role in other diseases, such as polycystic ovary syndrome. These pleiotropic effects may prove to be clinically relevant. There has been recent debate about the possible differences between the two thiazolidinediones in terms of cardiovascular disease outcome. In this context, differences in the lipid effects between the two drugs may be relevant.