Patients with masked hypertension show normal office blood pressure but elevated out-of-office blood pressure. Out-of-office blood pressure is evaluated by ABPM (ambulatory blood pressure monitoring) or HBPM (home blood pressure measurements). HBPM is more popular in Japan because its simplicity. However, ABPM is essential to measure night blood pressure. Patients with masked hypertension have increased cardiovascular morbidity risk as high as seen in those with established hypertension. Many factors, including life style (smoking, excessive alcohol drinking, etc), variability of blood pressure (morning surge, non-dipper, riser) and inappropriate treatment of hypertension, are involved in masked hypertension. Life style modification, strict and sophisticated blood pressure control, and treatment of underlying diseases are necessary to treat masked hypertension.