An overview of the 17 completed randomised trials of antihypertensive treatment demonstrates that a 5-6 mm Hg reduction in DBP reduced stroke risk by 38% (SD 4) and CHD risk by 16% (SD 4). These results indicate that a few years' treatment with diuretic- or beta-blocker-based therapy produces most or all of the long-term stroke avoidance and much of the long-term CHD avoidance that would be predicted from observational epidemiological studies, given the blood pressure reductions that were achieved in the trials. The relative risk reductions were similar in trials of older and younger patients, although the absolute reduction in events was more than twice as great in the trials in older patients. From these results it can be estimated that in fully compliant patients at similar risk of vascular disease to those included in the trials, antihypertensive treatment for 5 years would prevent one major vascular event among every 20 older patients and one major vascular event among every 60 younger patients. Obviously the benefits of treatment will be greater among those at higher risk than the patients included in the previous trials. The greatest benefits are likely to be achieved in those with a history of vascular disease since their risk of future events is particularly high. Among such patients it is possible that blood pressure reduction will confer worthwhile benefits in those without hypertension, as well as those with hypertension. It is also possible that the benefits of treatment will be determined by the size of the blood pressure reduction and by the choice of the anti-hypertensive agent. However, each of these possibilities requires confirmation in large scale randomised controlled trials.