A metaanalysis of the major clinical trials on the benefits of antihypertensive therapy revealed that there is an overall reduction of coronary events by treatment, but that it is only about one-half of the reduction predicted. The reduction in stroke achieved the predicted number. This "failure" has been attributed to study design, patient age at entry, preexisting disease, high-lipid diets, drug-associated cardiac dysrhythmias, and the "J-curve" effect. It is also possible that the drugs used in these trials did not suppress the morning acceleration of blood pressure during which most coronary and cerebrovascular events occur. Ambulatory blood pressure monitoring studies suggest that antihypertensive drugs reduce overall blood pressure throughout the 24-h period, but do not change the circadian curve. The beneficial effect of antihypertensive therapy probably rests with the magnitude of blood pressure reduction. It is unlikely that lack of suppression of morning acceleration alone accounted for the failure of "older" drugs to prevent as much cardiac mortality as expected.