This paper describes the design of the West of Scotland Coronary Prevention Study (WOSCOPS) which is a primary prevention trial involving men aged 45-64 yr with raised plasma cholesterol levels. The principal aim is to test the hypothesis that reduction of serum cholesterol by treatment with pravastatin [a competitive inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase--a regulatory enzyme in cholesterol synthesis] over an average period of 5 yr will lead to a reduction in fatal and non-fatal myocardial infarction. A trial population of approx. 6500 men have been randomized in equal numbers to treatment with placebo or pravastatin. At the time of randomization, these men have no evidence of previous myocardial infarction. All subjects are given smoking and dietary advice throughout the study. The principal endpoints are: (i) coronary heart disease death plus non-fatal myocardial infarction, (ii) coronary heart disease death, and (iii) non-fatal myocardial infarction.