Little information on the possible influence of obesity on diastolic function is available. Thus we studied 24 asymptomatic obese volunteers (greater than 120% ideal body weight; mean 153 +/- 30%), aged 34 +/- 11 years. Each obese subject was matched for age, height, and sex, with a healthy normal-weight control subject. Isovolumic relaxation time (IVRT) and diastolic filling indexes were determined by pulsed Doppler echocardiography. The IVRT was significantly prolonged in the obese group (84 +/- 17 msec) as compared with the control group (57 +/- 13 msec; p less than 0.0002). Multiple regression analysis showed that percentage of ideal body weight was the most important predictor of peak early filling velocity (r = 0.59, p less than 0.005) and mean deceleration rate of early filling (r = 0.61, p less than 0.005) in the obese group. However, age as compared with percentage of ideal body weight was a more important determinant of the relative distribution of early and atrial filling, such as peak early-to-atrial filling velocity ratio (r = -0.75, p less than 0.0001). Ejection fraction, heart rate, and blood pressure did not differ between the obese and control groups. In conclusion, obesity is associated with preclinical abnormalities of IVRT, which may reflect impaired relaxation. The IVRT may be useful in the early detection of left ventricular dysfunction in obesity. Last, studies comparing Doppler indexes of diastolic filling among groups must control for potential differences in percentage of ideal body weight.