After a pulmonary arterial banding procedure the phonocardiograms of 38 patients were correlated with haemodynamic and angiographic findings. Twenty-four patients had uncomplicated ventricular septal defect, 2 had single ventricle, 5 had transposition of the great arteries, 5 had atrioventricular canal defects, and 2 had coarctation of the aorta and ventricular septal defect. P2 was separated from A2 by less than 40 ms in 10 of the 11 patients with high pulmonary vascular resistance. Of 27 patients with nearly normal pulmonary vascular resistances and distal pulmonary artery pressures less than 50/20 mmHg (6-7/2-7 kPa), 24 had A2-P2 intervals of over 40 ms. A narrow A2-P2 interval with a satisfactory band was found in 2 patients with large right-to-left shunts. A2-P2 interval did not change appreciably with age, and this measurement is a useful guide as to the effectiveness of pulmonary artery banding by one year after operation. If this interval is less than 40 ms, repeat catheterization should be carried out as such patients may have persisting pulmonary hypertension and an increased pulmonary vascular resistance.