The morbidity and mortality associated with hypertension demonstrates the need to direct antihypertensive therapy towards reducing target organ damage, particularly those that are well validated predictors of high risk. It is now well recognized that closer relationships exist between hypertensive organ disease and blood pressure from 24-h ambulatory blood monitoring as opposed to conventional clinic determinations. Furthermore, two further important features have been apparent in studies using 24-h blood pressure measures. First, patients who fail to exhibit a "normal" nighttime circadian fall in blood pressure show a higher incidence of cardiovascular and cerebrovascular complications. Second, there is convincing evidence to suggest that blood pressure variability over a 24-h period is an important independent determinant of target organ damage. This evidence suggests that optimal benefit will result from the use of drugs that effectively lower blood pressure over a full 24 h dosage interval, and that this smooth blood pressure effect should be superimposed upon the normal circadian blood pressure pattern and at the same time reduce blood pressure variability. It is likely that this will only be achieved by the use of genuine long-acting antihypertensive drugs with a high trough:peak ratio.