Cases of coexisting Cushing's disease and unilateral adrenal tumor are rare, and there are conflicting reports in the literature regarding both the pathophysiology and the incidence. We have reported the case of a patient with a biochemically inactive incidental adrenal mass in whom pituitary-dependent Cushing's syndrome developed during follow-up. Authors of past reports have suggested that transitional change is responsible for the adrenal disease. A complete review of the literature has shown that cases of coexisting pituitary Cushing's disease and unilateral adrenal tumor, although rare, are more common than was previously believed. After studying a case in which the adrenal mass was present before disease developed, we suggest that the coexistence may be incidental in some cases.