T HE pbeocbromocytoma is a tumor of the chromaffrn tissue of the body; and since chromaffrn tissue is rather widely distributed, it follows that the location of these tumors may also vary widely. Naturally, since most of the chromaffrn tissue is in the adrenal meduIla, this is by far the most common location for these tumors. However, they may occur along the aorta, in the region of the sympathetic ganglions, in the organ of Zuckerkandl at the bifurcation of the aorta, and in such widely diverse locations as the top of the mediastinum and the adductor canal of the thigh. It is important to point out also that approximately IO per cent of these tumors are reported to be multiple. Therefore the finding of one does not exclude the possibility that there may be another functioning tumor elsewhere. It is also worth reporting that at least IO per cent of these tumors are malignant. The importance of these tumors is that they may produce epinephrine and norepinephrine in large quantities. The clinical symptomatology appears to vary depending on the relative amounts and the action of these two chemicals reIeased into the blood by the tumors. Varying quantities of epinephrine and norepinephrine have been successfuI1y extracted from these tumors many times. The diagnosis is to be considered when symptoms which may be caused by the action of epinephrine and norepinephrine are encountered. The symptoms most common are those associated with vascular and blood pressure phenomena. CIassicaIly, the description is that of hypertension in episodes-in “attacks,” the patient usually wiI1 say. Associated with these attacks are headache, palpitation, sometimes extreme sweating, nausea and vomiting, and sometimes syncope and exhaustion. This is the cIassic picture, and it is the picture with which the tumor was first associated. However, it has become apparent over the Iast ten or fifteen years that the tumor can aIso produce sustained unremitting hypertension. Therefore, some people have gone so far as to suggest that some of the diagnostic tests I shall describe should be used in every instance of serious essential hypertension. Sweating is the next most commonIy reported symptom according to the literature. Vasomotor phenomena are common. These consist of blanching alternating with rubor of the skin of the face or sometimes of the extremities. They probabIy have to do with variations in the amount of epinephrine and norepinephrine produced. An eIevated temperature has been reported rather consistently. There is often associated diabetes, or at least glycosuria. As is commonly known, one of the effects of epinephrine is the production of an elevation in blood sugar. The fasting bIood sugar is quite commonly above 120 mg. per cent. Another common and often deceiving symptom is a genera1 eIevation of oxygen requirement and, therefore, eIevation of the “basal” metabolic rate which may be mistaken for Graves’ disease. In certain instances thyroidectomy has been carried out, unsuccessfully of course. In other instances unsuccessful antithyroid therapy has been instituted. Just as a diagnostic point, it is worth mentioning that the protein-bound iodine is not elevated in patients with pheochromocytoma and that fact should be an important diagnostic distinction. Finally, one of the important symptoms is autonomic imbalance invoIving smooth muscle functions. These patients may become very il1 with ileus and serious abdominal clistention. Apparently they are also more prone to sudden death, particularly foIIowing operations or other situations of stress. These, then, are the symptoms which must be kept in mind if one wishes to pick up these tumors. There are certain provocative tests which may be used in instances in which this