Introduction: A number of randomised trials have been initiated to address the current uncertainty surrounding the efficacy of newer blood pressure lowering agents. Many of these may not be individually large enough to detect plausibly modest differences between treatments.

Objective:

* To establish a register of all major ongoing trials of blood pressure lowering agents;
* to conduct overviews to determine the separate effects of angiotensin converting enzyme inhibitors and calcium channel blockers compared with diuretic or beta-blocker-based therapy, on the risks of stroke and coronary heart disease.

Methods:

A register of major ongoing and planned randomised trials has been established and collaboration sought from the principal investigators of each study. A detailed protocol has been prepared for overviews of individual patient data with anayses scheduled for 1999 and 1996.

Results:

29 eligible trials have been identified and characterised. These trials will provide a total of approximately 1,000,000 patient years of follow-up from 200,000 randomised patients. It is anticipated that more than 5,000 strokes and 10,000 coronary heart disease events will be documented.

Discussion:

The overviews planned should have good power to resolve the principal persisting uncertainties about the relative efficacy of newer and older agents. The prospective design and the use of individual patient records will overcome the problems associated with retrospective overviews based on tabular data.