The British Regional Heart Study seeks to define risk factors for cardiovascular disease, to examine their interrelationships, and to explain the geographic variations in cardiovascular disease in Britain. A clinical survey of men aged 40-59 in 24 British towns was carried out and preliminary data from the survey analysed. On a town basis cardiovascular mortality was associated with mean systolic blood pressure and the prevalence of heavy cigarette smoking and heavy alcohol consumption. No such association was seen for body mass index or mean serum total cholesterol or high-density-lipoprotein cholesterol concentration. Cigarette smoking and alcohol intake and, to a less degree, systolic blood pressure were related to the social class (percentage of manual workers) of a town, and these factors may determine to some extent the increased risk of cardiovascular disease in manual workers. Blood pressure in individual subjects was affected predominantly by age, body mass index, and alcohol intake. Body mass index appeared to affect blood pressure to a greater extent than alcohol intake and did so with a consistent and positive linear trend. Nevertheless, the differences between towns in mean blood pressure readings appeared to be more closely associated with variations in the prevalence of heavy drinking than with variations in body mass index. Alcohol intake and body mass index explained only a part of the striking differences between towns in mean blood pressure readings, and some important "town"factors remained unexplained.