Since pheochromocytomas result in a curable form of hypertension, there should be an awareness of its diagnostic features. These include the following: paroxysmal hypertension, paroxysmal vasomotor phenomena, hypermetabolism with loss of weight or elevation of the basal metabolic rate, and “diabetes” with hyperglycemia and glycosuria. We have studied twelve patients with pheochromocytoma. Four of them were under 25 years of age, four from 26 to 50 years, and four from 51 through 65 years. Seven were women and five were men. Five of the patients had both diabetes and hypermetabolism, two had diabetes without hypermetabolism and three had hypermetabolism without diabetes. Only two of the patients had neither diabetes nor hypermetabolism. Intravenous pyelograms were performed on eleven of the patients, and accurately localized a lesion in only two. Retroperitoneal carbon dioxide studies were performed in nine cases. They accurately localized the lesion in four cases and were of some help in localizing it in three others. The phentolamine test was positive in eleven patients, and in the twelfth one, the benzodioxane test was positive. The importance of the diagnostic tests utilizing phentolamine and catecholamine excretion has been emphasized and the pitfalls in their use have been reviewed. Stress was placed on the importance of having urine specimens that are timed and measured, in order to prevent false positive catecholamine excretion values which may result from concentrated urine and falsely negative values from dilute urine. The determination of the conjugated 3-methoxy metabolites of epinephrine and norepinephrine (metanephrine and normetanephrine) have been diagnostic in all six cases in which they have been determined. The radiographic localization of the tumor by the combined use of retroperitoneal carbon dioxide and the intra venous pyelogram is of value to the surgeon. Correcting the blood volume abnormalities greatly simplifies the operative and postoperative management. Although this can be done by transfusion, the preoperative use of oral phentolamine is preferable because it is safer and more physiologic.