Testosterone is the principal sex hormone in men. It is important not only for normal sexual function but also for maintaining bone and muscle strength, mental and physical energy, and overall well-being. Low testosterone is associated with diminished libido, erectile dysfunction, increased fat mass, decreased muscle, bone mass and energy, depression, and anaemia. Type 2 diabetes may be one of the commonest causes of  hypogonadism – a lack of function in the testes, which adversely affects testosterone production. The authors report on diabetes-related hypogonadism and its wider effects, and call for further efforts to understand and prevent this growing problem.