Abstract Associations between body mass index (BMI) and blood pressure (BP) have been consistently observed, but remain poorly understood. One unresolved question is whether there is a linear relationship across the entire BMI range. We investigated this question among 11 235 adult men and women from seven low-BMI populations in Africa and the Caribbean. We used kernel smoothing and multivariate linear and spline regression modeling to examine gender differences in the relationship and to test for a threshold. Age-adjusted slopes of BP on BMI were uniformly higher in men than women, with pooled slope ratios of 2.00 and 2.20 for systolic and diastolic BPs, respectively. Men displayed no evidence of age modification or nonlinearity in the relationship, and the age-adjusted slope of systolic BP on BMI was 0.90 (95% confidence interval [CI], 0.76 to 1.04). Women demonstrated both age modification and nonlinearity. For both younger (