Eleven children who received transplants at less than 2 years of age underwent 59 echocardiograms at the time of endomyocardial biopsy for the assessment of the ability of echocardiography to predict acute rejection in the infant heart transplant recipient. Two patients died of acute rejection and autopsy findings were compared with premortum echocardiograms. Biopsy specimens were graded as no rejection (n = 46), mild rejection (cellular infiltrate, n = 5), or moderate-severe rejection (myocyte necrosis/edema, n = 8). Echocardiographic indexes measured included the following: left ventricular mass, left ventricular volume, ejection fraction, heart rate, and peak rate of posterior wall thinning. Compared with controls, patients during mild rejection had slower posterior wall diastolic thinning (p < 0.01). No significant change was noted in left ventricular mass until endomyocardial biopsy specimens showed severe rejection. No significant changes were noted in heart rate or ejection fraction in any of the groups. In conclusion, decrease in the peak rate of posterior wall diastolic thinning may be a sensitive indicator of acute rejection in the infant heart transplant recipient.