To evaluate body weight as a determinant of cardiac structure and function in adolescents, 47 normotensive 13-year-old subjects were studied with M-mode echo duplex Doppler echocardiography (DE). They were separated by body weight into standard (greater than 55 kg) and lighter (less than 55 kg) groups. No subject had a blood pressure greater than 140/90. The differences between the groups in systolic (SBP) and diastolic (DBP) blood pressures, LV mass, left atrial size (LA), and peak aortic flow velocity (PAoV) were measured. (Formula: see text). Body weight strongly correlated with SBP in the entire study group (r = 0.66, P less than 0.001) but slightly less strongly with DBP (r = 0.48, P less than 0.001). Left ventricular filling indices such as peak early diastolic flow velocity, early diastolic filling integral, peak atrial flow velocity, and atrial filling integral were not significantly different between the groups. No significant differences in blood pressure, LV mass, LA size, or peak aortic flow velocity could be demonstrated when the subjects were subdivided on the basis of gender or presence or absence of family history of hypertension. Body weight appeared to be not only a strong predictor of blood pressure but also of cardiac structural and functional characteristics in adolescents.