Diagnosis of corticotropin-producing bronchial carcinoid tumors causing Cushing's syndrome Cushing's syndrome due to ectopic production of adrenocorticotropic hormone (corticotropin) has been recognized for many years. Traditionally, clinicians have thought that most cases were due to lung carcinomas and that the clinical manifestations differed from those for pituitary-dependent Cushing's syndrome. We report two cases of corticotropin-producing bronchial carcinoid tumors that were clinically and biochemically indistinguishable from pituitary-dependent Cushing's syndrome. Review of the literature revealed that bronchial carcinoid tumors are the most common cause of Cushing's syndrome due to ectopic secretion of corticotropin. On biochemical and anatomic studies, they are frequently indistinguishable from pituitary-dependent Cushing's syndrome and thus may be difficult to diagnose. Inferior petrosal sinus sampling for corticotropin and computerized imaging of the chest may be the best aids in making the diagnosis. 