Cushing’s syndrome, primary aldosteronism, unilateral renal disease (rarely, bilateral renal artery stenosis), and pheochromocytoma often produce hypertension. Hypertension due to these diseases is classified as secondary hypertension, in contrast to primary idiopathic (essential) hypertension. Although patients with these particular diseases that cause secondary hypertension are a relatively small minority of hypertension patients, the diseases are important because they are surgically curable. The patient usually is protected against the bad effects of hypertension by early diagnosis and cure. Patients who must be especially investigated are those who are young (less than age 50 years), those whose symptoms develop over a short time, or those who have a sudden worsening of their hypertension after a previous mild stable blood pressure elevation.