The definition of late onset hypogonadism in the aging male is controversially debated, and according to the latest literature consists of at least three especially sexual symptoms such as loss of morning erection, low sexual desire and erectile dysfunction as well as a total testosterone <8-11 nmol/l. Testosterone replacement therapy in the aging male has been shown to have a beneficial effect on muscle and fat mass as well as on bone mineral density, with more conflicting effects observed on muscle strength, sexual function, mood and quality of life. The prescriptions for testosterone products for the aging male increased by over 170% in the previous five years. Furthermore, there is a lot of epidemiological data showing an inverse relationship between testosterone levels and obesity, insulin resistance, the metabolic syndrome and type 2 diabetes mellitus. However, only few small randomised placebo-controlled studies have investigated the effect of testosterone replacement therapy on insulin resistance and HbA1c levels, with controversial results. Importantly, so far the long-term safety and efficacy of testosterone replacement therapy has not been established. Although until now no clear evidence has been found that testosterone replacement therapy has a causative role in prostate cancer or indeed in changes of the biology of the prostate, in a recent meta-analysis a 4-fold increased risk of prostate-associated event rates in testosterone treated elderly men sounds a note of caution. Also the risk for cardiovascular events is still not clear and caution is warranted especially in elderly men with cardiovascular disease and limited mobility. In summary, the actual available evidence of long-term risks and outcome of testosterone replacement therapy is still very limited and carefully designed placebo-controlled trials of testosterone administration to assess the risks and benefits of such a therapy are required. Until then, testosterone treatment in elderly men should be restricted to elderly men with clearly low testosterone levels in the presence of clinical symptoms, and the advantages and disadvantages need to be accurately weighted. A careful monitoring of potential side effects is necessary.