This communication is based on a review of the clinical histories of 100 patients with proved pheochromocytoma seen at the Mayo Clinic between 1945 and 1965. The tumor was identified at operation in 97 patients and at autopsy in three. In 85% of cases only one adrenal gland was involved. Symptoms occurred almost always paroxysmally, the commonest symptom combination being headache, perspiration, palpitation, and pallor. The attacks were sudden in onset, generally less than one hour in duration, and of variable frequency. Basal metabolic rate and sugar concentration in the blood were increased in more than half of the patients. The neurological conditions most likely to be confused with pheochromocytoma include vasodilating headache, intracranial tumor, diencephalic-autonomic epilepsy, hypertensive encephalopathy, focal arterial brain disease, and anxiety state.