In contrast to women, men do not experience a sudden cessation of gonadal function comparable to menopause. However, there is a progressive reduction in hypothalamic-pituitary-gonadal (HPG) function in aging men: testosterone (T) levels decline through both central (pituitary) and peripheral (testicular) mechanisms and there is a loss of the circadian rhythm of T secretion. In cohorts of men 75 years of age, mean plasma T levels are 35% lower than comparable young men, and more than 25% of men over 75 appear to be T-deficient. Such age-associated T deficiency, which has been termed 'andropause', is thought to be responsible for a variety of symptoms experienced by elderly men, such as weakness, fatigue, reduced muscle and bone mass, impaired haematopoiesis, oligospermia, sexual dysfunction, depression, anxiety, irritability, insomnia and memory impairment. However, it has been difficult to establish correlations between these symptoms and plasma T levels. Nevertheless, there is some evidence that T replacement leads to symptom relief, particularly with respect to muscle strength, bone mineral density, and haematopoiesis. Studies to date on the specific association between psychiatric symptoms, such as depressed mood, and T levels have been methodologically flawed. Overall, data suggest that although hypogonadism is not central to major depressive disorder (MDD), HPG hypofunction may have aetiological importance in mild depressive conditions, such as dysthymia.