Plasma cortisol was measured at 20-min intervals for 24 h in eight patients with Cushing's disease and ACTH-secreting pituitary tumors. The 24-h mean (+/- SD) cortisol level was 25.6 +/- 11.3 microgram/dl (range, 15.5--40.6), which was significantly higher than the level of normal control subjects (P less than 0.01). The 24-h mean ACTH level varied between 22--107 pg/ml, with a mean +/- SD of 63.4 +/- 27.2. The mean ACTH level was higher than that of the control subjects but the difference was not statistically significant. The 24-h cortisol secretory pattern was characterized by an absence of the normal circadian variation and a failure of the plasma cortisol level to fall to less than 2 microgram/dl between 2300--0300 h. The coefficient of variation, an expression of the amplitude of cortisol secretory episodes, was significantly decreased in patients with Cushing's disease compared to normal control subjects; there was no significant difference in the number of cortisol secretory episodes in the patients vs. control subjects. Three of the patients were restudied after successful resection of their ACTH-secreting pituitary tumors. Two showed normalization of their 24-h circadian cortisol patterns and normal metyrapone responses. In the third, the 24-h mean cortisol level was normal, but the circadian cortisol rhythm remained abnormal. This patient had diminished ACTH reserve, demonstrated by a subnormal response to metyrapone. Additional studies will be required to determine if normalization of the circadian cortisol rhythm occurs in all patients with Cushing's disease who are cured after transsphenoidal microsurgery and who also show normal ACTH reserve.