REVIEW IN DEPTH: In recent years noninvasive ambulatory blood pressure monitoring has begun to find acceptance as a clinically diagnostic tool. Several national organizations, from Brazil, France, Italy, Switzerland, and the USA, have published guidelines concerning when and how it should be used. With one exception, all advocate its clinical use in selected categories of patients, including those with 'white-coat' hypertension, autonomic dysfunction, episodic hypertension, and refractory hypertension. None recommends it as a routine test. Monitoring for 24 h is generally recommended, with readings taken every 15-30 min. The area of greatest disagreement among the guidelines is the upper limit of normal ambulatory blood pressure.