Objectives: To determine the relationships between body mass index (BMI) and diet, social and behavioural factors among adult Jamaicans of African origin. Design: Cross-sectional. Setting: Urban communities in Jamaica, West Indies. Subjects: Three-hundred and sixty-three males and 561 females of African origin, aged 25 -74 y. Results: Women had higher mean BMIs (27.5 AE 6.4 kg=m 2 ) than men (23.4 AE 4.3 kg=m 2 ); 30.7% of women compared with 6.7% of men were obese. There was a tendency for obese men to have higher percentage of intakes from fat and less from carbohydrate, and women reported diets in which the percentage contribution of protein increased significantly with increasing BMI. In multivariate analyses, BMI was not explained by energy but was associated with protein intakes in females only. Predictors of relative weight were inversely related. Social (marital status) and behavioural (cigarette smoking) factors predicted BMI in both genders; older age in men and increased fibre intakes in women were associated with lower BMI. Conclusions: Social and behavioural factors are important determinants of body weight. Further investigations are needed which consider factors such as physical activity, genetic and other environmental variables as predictors of relative weight.