Several new drugs have become available for the treatment of type 2 diabetes mellitus in the past few years, and among them the thiazolidinediones are probably the most promising and interesting. Their mechanism of action involves a reduction in insulin resistance while simultaneously improving some of the independent risk factors for cardiovascular disease frequently found in patients with type 2 diabetes mellitus. On the other hand, while they might have many advantages over the other available antihyperglycemic agents, there are still some concerns about their long-term safety. Consequently, while awaiting the results of the planned long-term cardiovascular outcome studies, which will help establish their true benefits and risks, physicians must remain skeptical about these drugs and consider not only their claimed advantages, but also their not-so-well-publicized risks. This article summarizes the known and/or presumed beneficial and toxic effects of these drugs.