We report a case of Cushing's syndrome due to bilateral adrenal adenomas. A 45-year-old woman was found to have Cushing's syndrome during the course of treatment for diabetes mellitus. The diagnosis of Cushing's syndrome was based on the absence of a diurnal rhythm in plasma cortisol and failure to suppress plasma cortisol by 1 or 4 mg of dexamethasone. The plasma level of adrenocorticotropic hormone (ACTH) was below the normal range, and plasma cortisol responded normally to rapid ACTH injection. Abdominal computed tomography revealed bilateral adrenal tumors. Bilateral uptake of radiocholesterol by the adrenal cortex was observed in adrenal scintigraphy. Bilateral adrenalectomy was performed. Microscopic examination and analysis of steroid contents by high performance liquid chromatography showed that the tumor was cortisol-producting adenoma.