We are studying the contribution of risk and genetic factors, and their interaction, to the development of ischaemic heart disease (IHD) and other cardiovascular endpoints.The study is prospective, based in three centres in the south, east and north of France and in Northern Ireland. A total of 10 592 men aged 50-59 years were recruited from 1991 to 1993, and examined for evidence of IHD at baseline. Subjects are followed annually by questionnaire. Clinical information is validated from hospital and GP records. Demographic characteristics were similar in all four centres. Body mass index was highest in Strasbourg (mean 27.4 kg/m2 vs. 26.3 kg/m2 in Toulouse and Belfast), but total cholesterol, triglyceride and fibrinogen were highest in Belfast. In Belfast, 6.1% reported having had a coronary angiogram, compared to 3.0% in Toulouse. Conversely, 13.8% in Toulouse reported taking lipid-lowering drugs vs. 1.6% in Belfast. As predicted, a history of myocardial infarction (MI) was highest in Belfast (6.1%) and lowest in Toulouse (1.2%). Some 7.1% of Belfast men reported a medical diagnosis of angina vs. 1.5% in Toulouse. Subjects showing evidence of pre-existing IHD will be studied prospectively but treated in the analysis as an additional variable. These results provide a measure of reassurance that these cohorts are representative of the communities from which they are drawn and provide a reliable baseline for prospective evaluation and cross-sectional comparisons. The levels of the classical risk factors found in this study, particularly when examined in combination, as multiple logistic functions based on previous British studies, are very similar between centres and cannot explain the large differences in the incidence of IHD which exist. Additional risk factors may help explain, at least in part, the major differences in incidence of IHD between these study centres.