Ambulatory blood pressure monitoring enables the recording of the circadian rhythm of blood pressure under everyday circumstances, with the majority of individuals displaying diurnal variations in both systolic and diastolic blood pressures. During sleep, blood pressure in most people is between 10% and 20% lower than the mean daytime value. On arousal and the start of day-to-day activities, there is a surge in blood pressure that may last for between 4 and 6 h. Extensive evidence shows that ambulatory blood pressure is superior to office values in predicting cardiovascular risk. Cardiovascular events, such as myocardial infarction, ischaemia and stroke are more frequent in the morning hours, soon after waking, than at other times of day. Circadian variations in biochemical and physiological parameters help to explain the link between acute cardiovascular events and the early morning blood pressure surge. Recent observations in elderly Japanese individuals demonstrate that greater early morning blood pressure surges are related to an increased incidence of overt cerebrovascular disease; individuals with the greatest increases in blood pressure on awakening also had the greatest prevalence of silent ischaemic events and were more likely to experience multiple infarcts. Antihypertensive drugs that provide blood pressure control at the time of the early morning surge should provide greater protection against target-organ damage and enhance patient prognosis. Ambulatory blood pressure monitoring may be particularly helpful in assessing the circadian pharmacodynamics of such antihypertensive drugs. The technique has demonstrated, for example, a significantly greater reduction in blood pressure for the last 6 h of the 24-h dosing interval with telmisartan compared with valsartan.