The influence of an exercise intervention program on cardiac dimensions was studied in 79 normal children (aged 6 to 7 years) in an experimental (n = 38) and control (n = 41) group. The experimental group participated in an aerobic exercise session that met four days/week for 8 months. Anthropometric measurements and M mode echocardiograms were obtained before and after the intervention program. Comparison of the data between groups revealed no significant (probability [p] greater than 0.05) differences in left ventricular end-diastolic dimension, shortening fraction or resting heart rate. Left ventricular posterior wall thickness exhibited a significant increase (p less than 0.0256) from 3.9 to 4.7 mm in the experimental group compared with an increase from 4.3 to 4.6 mm in the control group after correcting for preintervention differences with an analysis of covariance. Likewise, left ventricular mass increased significantly (p less than 0.0004) from 21.2 to 27.4 g in the experimental group compared with 23.4 to 25.8 g in the control group. These findings indicate that when compared with control subjects, young children involved in an aerobic exercise program showed progressive increases in left ventricular posterior wall thickness and left ventricular mass and no change in left ventricular end-diastolic dimension, shortening fraction or resting heart rate.