We have carried out a prospective survey of 25 cases of male hypogonadism attending one hospital, and a retrospective study of 73 men attending other endocrine clinics in Manchester. In total, 47 had pituitary disorders, 15 isolated gonadotrophin deficiency (including 4 with Kallmann's syndrome), 10 testicular atrophy of unknown cause, 12 testicular damage, 10 with Klinefelter's syndrome, and 4 had miscellaneous disorders. Our survey emphasises the importance of adequate history and examination. Most patients presented with reduced libido, with marital problems in 62% of married men. Less common problems were facial flushing, osteoporosis and gross obesity. Several patients with pituitary disorders were asymptomatic, even in the presence of visual field defects. Klinefelter's syndrome, and testicular atrophy, may present with infertility or gynaecomastia rather than symptoms of androgen deficiency. On examination, the presence of gynaecomastia or obesity were of no help in differential diagnosis, whereas visual field defects clearly indicated a pituitary cause. Measurement of height/span was of little help. The precise diagnosis was usually established with basal plasma LH, FSH, testosterone and prolactin, with karyotype and pituitary radiology, and without more elaborate dynamic hormone tests. Testosterone esters given by intramuscular injection as "Sustanon 250" was the most commonly used replacement therapy. Improved libido usually resulted. Side-effect occurred in 10%, usually as muscle cramps, pain at the injection sites, acne, or excessive sex drive. One tragic case illustrates the potential dangers of androgen replacement therapy in an unrecognised psychopath, and where doubt exists a psychiatric opinion should be sought before starting therapy.