Summary 　It is well known that testosterone, which decreases with aging, plays many physiological roles in various organs. Late-onset hypogonadism （LOH） has received widespread attention in the last few years. Metabolic factors as well as metabolic syndrome are closely associated with decreased serum testosterone. In general, it is believed that testosterone is inversely related to adiponectine level, which is thought to be a key molecule in the etiology of metabolic syndrome. However, no inverse correlation exists between them in patients with LOH. Testosterone replacement treatment is expected to be one of treatments for metabolic syndrome. Furthermore, testosterone may be an important treatment tool as anti-aging.