Of hypertensive patients, 95% have primary (essential) hypertension. The remaining 5% of adults with hypertension have an identifiable or "secondary" disorder responsible for the elevated blood pressure. Although relatively rare, the diagnosis of secondary hypertension is important as these forms require specific treatments based on the underlying pathophysiology and are potentially curable. Secondary causes of hypertension include renal disease, such as chronic parenchymal disease and renovascular hypertension, a number of endocrinopathies, such as primary aldosteronism, pheochromocytoma, or thyroid disease, as well as a variety of miscellaneous causes including obstructive sleep apnea and substance abuse. An evaluation for secondary causes in all hypertensive patients is not necessary or cost effective. Clinical clues obtained from a careful history, physical examination, and laboratory assessment guide the clinician through an appropriate evaluation for identifiable causes. A combination of the clinician's index of suspicion and interpretation of data provide guidance in the choice of diagnostic tests and therapies to target the mechanisms contributing to poor blood pressure control. (c)2000 by Le Jacq Communications, Inc.