A 43-year-old woman presented with obesity and lumbago. Endocrinological examinations revealed normal plasma cortisol levels and a suppressed serum adrenocorticotropic hormone (ACTH) level. On venous sampling, markedly elevated plasma cortisol levels were observed for bilateral adrenal veins (243 and 62.3 micrograms/dl on the right and left sides, respectively). Although the computed tomogram revealed bilaterally enlarged adrenal glands, 131I-adosterol scintigram showed a strong uptake only on the right side. Right adrenalectomy successfully relieved Cushing's syndrome. Pathological diagnosis was adrenocortical adenoma, 3.5 cm in diameter. Cushing's syndrome recurred in 9 years. At that time, she underwent left subtotal adrenalectomy including a 3-cm adrenocortical adenoma. Postoperative convalescence has been uneventful with oral steroid supplementation. All 14 previously reported cases of bilateral adrenocortical adenoma (BAA) causing Cushing's syndrome as well as the present case were concurrent and dominant in females of reproductive age. This suggests that some cofactors other than ACTH, such as estrogen, contribute to the pathogenesis of BAA.