Pulmonary arterial pressure-flow relation was studied in 142 patients with artrial septal defect (ASD) and was compared with that of 139 patients with ventricular septal defect (VSD) and was compared with that of ductus arteriosus (PDA). The incidence of pulmonary arterial mean pressure (PAm) over 25 mmHg was 21 percent in ASD, 36 percent in VSD and 43 percent in PDA, and that over 40 mmHg was 3 percent, 23 percent and 19 percent, respectively. Large left to right shunt over 50 percent was found more frequently in ASD (58 percent) than in VSD (22 percent) and PDA (30 percent). When patients were separated into 2 groups at a PAm of 40 mmHg, lower pressure group showed a positive correlation between PAm and left-to-right shunt in each disease, although the correlation was poor in ASD (r equals 0.23) as compared with VSD (r equals 0.49) and PDA (r equals 0.47). The slope of the regression line was less steep in ASD (0.08) than in VSD (0.17) and PDA (0.14). It is considered that pulmonary hypertension in ASD develops on the basis of pulmonary vascular changes caused by prolonged hyperkinetic circulation.