Serum testosterone was measured pre- and post-operatively in 31 women who obtained remission of Cushing's disease following treatment by interstitial irradiation of the pituitary gland. The mean serum testosterone was initially elevated at 3.35 nmol/l, with values ranging from 0.7 to 14.4 nmol/l. Following treatment the mean serum testosterone fell to 1.42 nmol/l (i.e. normal) and all but one of the 15 patients with initially elevated pre-treatment values obtained normal levels. The pre-treatment testoterone concentrations correlated significantly (r = 0.47; p = 0.019) with the midnight ACTH concentration. The likely determinant of the raised serum testoterone would appear to be the intensity of the ACTH drive, although an individual's inherent sensitivity to ACTH may also be a factor.