Clinical studies with non-invasive ambulatory blood pressure monitoring have shown that some cardiovascular complications of essential hypertension (left ventricular hypertrophy, stroke) tend to be more frequent in patients whose 24-h blood pressure profile is flattened (non-dippers) and, consequently, suffer a longer duration of exposure to high blood pressure levels over the 24 h. The distribution of patients between dippers and non-dippers is conditioned by the limits of the blood pressure changes from day to night that are arbitrarily chosen to define these two groups, and by the time intervals defining daytime and night-time hours. Sleep does not seem to be disturbed by non-invasive monitoring to such an extent that the day-night blood pressure difference is affected. If daytime is defined as 0600-2200 h and night-time as 2200-0600 h, and those hypertensive patients with a nocturnal reduction in average daytime systolic and diastolic blood pressure of less than 10% are classed as non-dippers, the prevalence of non-dippers in essential hypertension appears to be about 35%. In these patients, left ventricular mass seems to be greater in non-dippers than in dippers among women, but not in men. The clinical significance of the dippers/non-dippers classification in the stratification of hypertensive patients of different levels of risk of cardiovascular complications needs further investigation.