The authors report five patients with adrenal cortical tumors in whom the preoperative diagnosis of pheochromocytoma was made. All patients had biochemical evidence of elevated catecholamine secretion in serum or urine. Clinically, two patients presented with symptoms suggestive of pheochromocytoma, and one patient had systemic hypertension that resolved following surgical excision of the tumor. Histologically, the findings were typical of adrenal cortical tumors: two adrenal cortical carcinomas and three adrenal cortical adenomas. Nevertheless, the immunohistochemical studies showed evidence of neuroendocrine differentiation in four tumors. Three tumors were positive for neuron-specific enolase and synaptophysin and a fourth tumor was positive for synaptophysin only. All neoplasms were negative for chromogranin. Electron microscopic studies in three tumors showed abundant endoplasmic reticulum and tubulovesicular cristae consistent with adrenal cortical cell origin. Neurosecretory granules, 150-300 mu in diameter, were found in one tumor. This current series of patients provides evidence that adrenal cortical neoplasms may be associated with clinical findings that simulate pheochromocytoma (so-called pseudo-pheochromocytoma). These clinical findings may be mediated by the presence of neuroendocrine features in these tumors.