Increased left ventricular mass has been shown to be a significant independent predictor of cardiovascular risk. The purpose of this study was to assess the separate and combined relations of obesity and hypertension with left ventricular mass and geometry. Echocardiographic findings in subjects in the Framingham Heart Study who were free of cardiopulmonary disease and were not taking cardiovascular medications were examined. M-mode studies that were adequate for estimating left ventricular mass were available in 624 men and 1,209 women. Height and weight measured at the time of echocardiography were used to calculate body mass index (in kg/m2), a measure of obesity. Casual sitting blood pressure measurements were obtained to detect rest hypertension. In subgroup analyses of lean normotensive, obese normotensive, lean hypertensive and obese hypertensive subjects, hypertension and obesity each had significant independent associations with left ventricular mass and wall thickness (all p less than 0.001 in men and women). Obesity was also associated with left ventricular internal diameter (p less than 0.001 in men and women). There were no synergistic influences of hypertension and obesity on any echocardiographic left ventricular variables. It is concluded that obesity and hypertension each have distinct associations with left ventricular mass and geometry. These strengths of association are additive but not synergistic.