A 49-year-old man with classic manifestations of Cushing's syndrome had undetectable levels of ACTH, lack of suppression of hypercortisolism with dexamethasone in doses of 2, 8, or 16 mg per day, bilaterally enlarged adrenal glands on MRI, and bilateral adrenal uptake of iodocholesterol. Preoperative treatment with ketoconazole lowered blood pressure and serum cortisol and produced symptoms of steroid withdrawal. Bilateral adrenalectomy revealed massively enlarged adrenal glands (left: 199 g, right: 93 g). Sequencing of the gene encoding the stimulatory G protein, GS alpha, did not show either of two activating mutations previously reported in patients with McCune-Albright syndrome or acromegaly. Twenty-three previous cases of Cushing's syndrome due to ACTH-independent massive bilateral adrenal disease (AIMBAD) have been reported. AIMBAD may cause confusion in the differential diagnosis of Cushing's syndrome as endocrine testing suggests a unilateral, ACTH-independent process while adrenal imaging demonstrates bilateral abnormalities. Bilateral adrenalectomy is curative and appears to carry little risk of Nelson's syndrome. The pathogenesis of AIMBAD appears to be heterogeneous, as recent reports have demonstrated GIP-mediated hypercortisolism and familial AIMBAD. Transition from Cushing's disease to ACTH-independence is not supported by the available data. Future cases of AIMBAD should be investigated carefully to further elucidate the pathogenesis of this disorder.