Masked hypertension has been drawing attention recently because this condition is often seen in untreated and treated individuals and is associated with target organ damage and a poor cardiovascular prognosis. Although masked hypertension is defined as normal office blood pressure with elevated ambulatory or home blood pressure, there are several subtypes. Morning hypertension is the most common form of masked hypertension, and is caused by natural circadian variation, evening alcohol consumption, and the use of short-acting antihypertensive drugs. Daytime hypertension may be caused by lifestyle factors such as habitual smoking and mental or physical stress. Nighttime hypertension is seen in various conditions that produce non-dipping status, including a high salt intake, renal dysfunction, obesity, sleep apnea, and autonomic failure. Advanced target organ damage such as increases in the left ventricular mass, carotid artery intima-media thickness, and urinary albumin excretion, is often present both in untreated and treated subjects with masked hypertension. In our study, the presence of the reverse white-coat effect is independently associated with those indices of organ damage among treated hypertensive patients. It is important to identify individuals with masked hypertension, to evaluate them with including the search for the subtype, and to treat each patient appropriately according to the cause of this condition.