The pre- and postoperative phonocardiograms of 24 children with uncomplicated atrial septal defect (ostium secundum type) were analyzed. The amplitude of the mitral (M1) and tricuspid (T1) components of the first heart sound were measured in each patient at the apex prior to surgery. It was found that in 18 of the 24 children studied (75 per cent) T1 had higher amplitude than M1 at the apex (T1M1 ratio > 1). Three out of 24 children (12.5 per cent) had equal T1 and M1 amplitudes (T1M1 ratio = 1), and 3 (12.5 per cent) had T1 amplitude less than M1 (T1M1 ratio < 1). No positive correlation was found between the T1M1 ratio, and the size of the left to right shunt, mean pulmonary artery pressure, right ventricular hypertrophy, and/or degree of splitting of the first heart sound. After surgery the T1M1 ratio became less than 1 in 22 children and equal to 1 in 2 children. A T1 higher than M1 at the apex is a sensitive index of atrial septal defect since this finding was present even in the presence of a small left to right shunt. Routine phonocardiographic measurement of the amplitude of both components of the first heart sound at the apex is recommended.