In the past few years non-invasive ambulatory blood pressure monitoring has become a widespread technology in the assessment of arterial hypertension. The "white coat effect" concept derives from comparison between "clinic" and "ambulatory" blood pressure. It consists of two opposite conditions: clinic hypertension with ambulatory normotension (the so-called "white coat hypertension") and clinic normotension with ambulatory hypertension (the so-called "white coat normotension"). Nearly 20% of unselected referred populations shows conditions that may not need medical treatment, such as the "white coat hypertensives", or that may need antihypertensive therapy, such as the "white coat normotensives".