Thiazolinediones are increasingly prescribed to improve glycaemic control in patients with type 2 diabetes. Experimental evidence suggests that these agents may exert a beneficial effect on cardiac haemodynamics, and protect against the development and progression of heart failure. However, these agents have been reported to precipitate heart failure, and in all cases this has been reversed by discontinuation of thiazolinediones, implicating a reversible mechanism. We report a unique case of irreversible dilated cardiomyopathy precipitated by pioglitazone, highlighting the potential for thiazolinediones to cause irreversible cardiac dysfunction. At present the underlying mechanisms for this are unclear, but this warrants further research.