Aging is the biggest risk factor of erectile dysfunction (ED). Seventy-one percent of Japanese men aged 70 to 79 years reported suffering from ED. Blood vessels and nerves related to erection are damaged by various risk factors as well as histological change in a corpus cavernosum penis, decreased testosterone level, and various complexed factors. Phosphodiesterase 5 inhibitors are the first-line choice for both on-demand and chronic treat- ment of ED. Late-onset hypogonadism (LOH) is diagnosed when declining testosterone con- centrations in the aging male cause ED and the metabolic syndrome (MetS). Testosterone replacement therapy for these patients has beneficial effect on ED and MetS and improves quality of sexual life.