Platelet catecholamine content was determined by radioenzymatic assay in 16 patients with elevated plasma norepinephrine or epinephrine or both and a clinical picture suggesting pheochromocytoma. Twenty-two normal subjects served as controls. Pheochromocytoma was documented in 10 patients, all of whom had markedly elevated platelet catecholamines. The other six patients had no tumor, and their platelet catecholamine levels were normal. Platelet catecholamines tended to return to normal more slowly than plasma catecholamines after removal of the pheochromocytoma, suggesting that platelets were rich in stored catecholamines. Determination of platelet catecholamine content is a helpful aid to the diagnosis of pheochromocytoma in patients with suggestive but not diagnostic elevations of the plasma catecholamine concentration.