Recent studies have confirmed that the mean blood pressure levels recorded during ambulatory monitoring are lower than the levels measured by the standard cuff method, possibly due to the "white coat" effect which involves as many as 25 to 30% of the total hypertensive population. In addition, ambulatory blood pressure monitoring is particularly useful to evaluate the day/night cycle. Loss of the normal decrease in blood pressure at night occurs in certain types of secondary hypertension and can be revealed by ambulatory monitoring. The importance of the morning peak in determining prognosis in patients suspected of having hypertension is illustrated by the large number of cardiovascular events which occur during this period. There is also a close correlation between the mean 24-hour blood pressure and cardiovascular prognosis. Studies have demonstrated that the loss of the circadian rhythm or the mean 24-hour blood pressure are significantly correlated with type I diabetes mellitus, the risk of cerebral vascular events and left ventricle hypertrophy. Ambulatory monitoring is also a particularly useful method in clinical trials. Reproducible results can be obtained in the same individual and there is no placebo effect, making it possible to reduce the number of participants and thus raise the statistical power of the test. Ambulatory monitoring can also modify management decisions as found in a recent survey conducted in the United States demonstrating the impact on the treatment of hypertension. For 30 to 40% of the hypertensive patients, therapeutic management was different after ambulatory monitoring than after conventional cuff measurement. Standardization will be the next step in the widespread use of ambulatory blood pressure monitoring.