The ageing process in men is marked by changes in body composition (loss of fat-free mass (FFM) and skeletal muscle, and gain in fat mass (FM)) and is associated with a decline in serum testosterone. Correlations between these aspects of ageing and the acknowledged role of exogenous testosterone in reversing the loss of FFM and gain in FM seen in adult men with congenital or acquired hypoandrogenism have led to the hypothesis that testosterone therapy in ageing men will result in favourable changes in body composition and may improve metabolic status and/or cardiovascular risk. Data from randomized controlled trials of testosterone therapy in ageing men addressing the endpoints of body composition and components of the metabolic syndrome and cardiovascular risk factors are reviewed, and the impact of the increasing prevalence of obesity on these relationships is considered.