Ambulatory blood pressure monitoring (ABPM) over 24 hours has become quite common. Evidence suggests that the mean 24-hour measurement is more closely associated with end organ damage, including end-points such as left ventricular hypertrophy (LVH), than is a single blood pressure measurement taken in the doctor's office. Clinicians disagree about the particular significance of blood pressure measurements obtained during exercise, blood pressure variability, and blood pressure load (a measurement above 140/90 over 24 hours). However, the morning peak in blood pressure appears to be associated with the highest incidence of coronary events, and end organ damage may be greater in subjects in whom nocturnal blood pressure falls only slightly from a diurnal baseline (non-dippers). From now, ABPM serves mainly as a research tool. Longitudinal controlled studies are needed to compare the value of ABPM to office blood pressure readings in terms of how these measurements can predict cardiovascular end-points or, more realistically, surrogate end-points, such as the development or regression of LVH.