Twenty patients with hypogonadism (19 women with amenorrhoea and 1 man with impotence and infertility), galactorrhoea and hyper-prolactinaemia (range: 36 to 344 ng/ml) were studied. The radiological study of the sella turcica, including in all cases hypocycloidal tomograms, allowed classification of the patients into 3 groups: group I (n = 4) had a grossly enlarged sella turcica, group II (n = 12) had localized alterations indicating the probable existence of a prolactin-secreting microadenoma ("microdeformation") while group III patients presented no radiological abnormality. Before treatment, all the patients were submitted to a complete evaluation of the function of their anterior pituitary, including the LH and FSH responses to iv administration of Gn-RH. All the group I patients had low basal LH levels and a blunted response to Gn-RH. The basal LH and in response to Gn-RH were normal in most of the group II patients and in all of the group III patients. An exaggerated FSH response to Gn-RH was observed in 6/12 patients with microdeformation (group II) but not in groups I and III patients. A low LH and a blunted 84 (1977) 738-749 Services d'Endocrinologie et de Médecine Generale, (Prof. M. Linquette et P. Fossati),