This paper describes a middle-aged man in whom an adrenal mass was incidentally discovered by upper abdominal echogram. Physical examination revealed no signs of Cushing's syndrome. The plasma cortisol level at 0800 h was within the normal range, but the diurnal rhythm had disappeared. Plasma ACTH was undetectable throughout the whole day. Urinary 17-OHCS was slightly increased and was not suppressed by 2 mg or 8 mg dexamethasone. Metyrapone test and CRF test revealed no response. A left adrenalectomy was performed and histological diagnosis of the removed tumor was an adrenal adenoma. After operation, oral steroid supplementation was necessary. These data suggest that the autonomous cortisol secretion by the tumor accounted for all his daily cortisol secretion, but it was too small to be clinically functional. We propose that every patient with an incidentally discovered adrenal mass should be subjected to endocrinological evaluations.