The relationship between socio-demographic and behaviour variables and body mass index (BMI: weight/height2) adjusted for age were studied in a population with high-normal blood pressure who participated in the Hypertension Prevention Trial. The BMI of the participants ranged from 19.1 to 35.1 kg/m2 in men and from 16.0 to 35.1 kg/m2 in women. The prevalence of obesity (BMI greater than or equal to 25.0 kg/m2) was 77 per cent in men and 61 per cent in women, with prevalence of severe obesity (BMI greater than or equal to 30.0 kg/m2) being 23 per cent and 19 per cent respectively. Stepwise regression was carried out to identify the most important correlates of BMI. In men, they were family income (+), occupation (-), leisure time exercise frequency (-), number of meals eaten out (-), alcohol intake (-), smoking (-), caffeinated drink intake (+), and meal planner. Men who planned meals with their partners had a higher BMI than men who planned their own meals or had someone else plan their meals. Correlates of little importance were marital status, race, education, number of members in household, energy intake, percentage of total calories from fat, occupation activity level, and vitamin/mineral supplement intake. In women the most important correlates of BMI were alcohol intake (-), caffeinated drink intake (+), and race. Black women had a higher mean BMI than white women. The important socio-demographic and behaviour variables in both men and women accounted for about 20 per cent of the variance in BMI which leaves about 80 per cent of the variation unexplained. This indicates the presence of other factor(s) which may be determining body weight.