BACKGROUND Previous studies have suggested that African Americans have a greater left ventricular relative wall thickness than whites with similar levels of blood pressure (BP), whereas other investigators have documented an attenuated nocturnal decline in BP and early remodeling of the resistance vessels in African Americans. METHODS To further evaluate the contributions of diurnal BP variation and vascular remodeling to ethnic differences in left ventricular geometry, we measured ambulatory BP, systemic hemodynamics, minimum forearm vascular resistance, and left ventricular structure in a biracial sample of 171 men and women between the ages of 25 and 45 years. RESULTS Despite similar resting BPs, African Americans had a greater indexed peripheral resistance, a greater minimal forearm vascular resistance, and a smaller nocturnal decline in BP than white subjects. African Americans also had a greater left ventricular relative wall thickness (0.41 +/- 0.07 v 0.38 +/- 0.08, P =.048) and a trend toward a greater indexed left ventricular mass (41.1 +/- 8.7 v 38.6 +/- 8.6 g/m(2.7), P =.087). Ethnic differences in relative wall thickness were no longer significant when adjusted for either indexed peripheral resistance (P =.173) or sleep systolic BP (P =.124). CONCLUSIONS In our cohort of young men and women, African Americans had a greater left ventricular relative wall thickness than whites with similar levels of resting BP. This early concentric remodeling of the left ventricle in African Americans may be mediated, in part, by hemodynamic influences, including a greater peripheral vascular resistance and a smaller nocturnal decline in BP.