Testosterone levels are reduced in obesity, the metabolic syndrome and type 2 diabetes. Low testosterone levels are now being recognised as an independent risk factors for these conditions. Findings from men undergoing androgen suppression as treatment for prostate cancer confirm that the hypogonadal state increases body fat mass and serum insulin and there is a high rate of developing new diabetes in this population. Clinical trial data are consistent in showing reductions in body fat mass during testosterone replacement therapy. There are also trials showing improvements in insulin resistance and glycaemic control with testosterone. Most of the trials in this area to date have been of small size and the promising results require confirmation in larger trials, which are underway. In the longer term, large trials should be conducted to assess the potentially beneficial effects of testosterone on cardiovascular risk in this and other patient groups. In the meantime physicians involved in the care of men with diabetes should remain vigilant for the symptoms and signs of hypogonadism. Testosterone replacement therapy should be considered for those men with subsequently confirmed hypogonadism.