ABSTRACT Plasma testosterone levels were suppressed in 6 of 8 mature male patients with Cushing's disease, all of whom complained of loss of libido and decreased sexual potency. Gonadotrophin levels, both under basal conditions and in response to LH-RH, were generally normal. The testicular response to stimulation with hCG was brisk in the 2 patients examined. Oestradiol levels were slightly elevated in 2 patients and prolactin levels were normal in all patients. Thus, male patients with Cushing's disease demonstrated normal gonadotrophin levels in the presence of suppressed testosterone, or, viewed from a slightly different prospect, low testosterone levels despite normal gonadotrophins. Neither oestradiol nor prolactin excess appeared to account for the observations. Possible explanations for these findings include (a) a combination of impaired hypothalamic and testicular function, and (b) a resetting downwards of the level of testosterone that is seen as appropriate by the disordered hypothalamic-pituitary unit. Following correction of cortisol excess in Cushing's disease, testosterone levels rose into the normal range.