BACKGROUND: Blood pressure is directly and causally associated with body mass index (BMI) in populations worldwide. However, the relationship may vary across BMI in populations of African origin. METHODS: We compared the relationship between blood pressure and BMI in populations of African origin, using 13 samples from Africa, the Caribbean, the United Kingdom and the United States. We had access to data from individual participants for age, height, weight, blood pressure, and treatment of hypertension. Analysis was restricted to 18,072 participants (age 35-64 years; 44% men). We carried out multivariate regression analysis to estimate the relationship between blood pressure and BMI by country and by sex. The use of antihypertensive treatment was taken into account by exclusion and by sensitivity analysis. RESULTS: There was a positive relationship between both systolic and diastolic blood pressure and BMI. In men the slopes for systolic blood pressure varied from 0.27 mm Hg per kg/m (95% confidence interval = -0.01 to 0.56) in the United States to 1.72 mm Hg per kg/m (95% confidence interval = 0.92 to 2.53) in Ghana (Kumasi). In women, the slopes varied from 0.08 (-0.54 to 0.72) in South Africa to 1.32 (0.98 to 1.66) in the Republic of Congo. Similar variation in trends was seen for diastolic blood pressure. The higher the BMI, the shallower the slopes [-0.10 (-0.15 to -0.06) for systolic, -0.09 (-0.12 to -0.06) for diastolic]. No differences were seen after excluding persons who were being treated for hypertension. CONCLUSIONS: Blood pressure and BMI levels vary among populations of the African diaspora. The effect of BMI on blood pressure levels diminishes as BMI increases. These results suggest a complex relationship among excess body weight, adiposity, and energy expenditure.