Preoperative and postoperative echocardiography were performed in 33 patients with isolated ostium secondum atial septal defect(ASD), confirmed by right heart catheterization and operation at Seoul National University Hospital from November 1980 to June 1982. The ratio of right ventricular enddiastolic dimension to leftventricular enddiastolic dimension(RVED/LVED) was compared to the pattern of interventricular septum, before and after operation. In patients with persistent paradoxical septal motion during 2wks after operation, repeated echocardiograms were performed upto 9 months, postoperatively. The results obtained are as follows: 1) The ratio of RVED/LVED in patients with ASD, was 0.95+/-0.35(mean+/-standard deviation), which was significantly larger than that of 14 normal subjects(p<0.005). 2) There was statistically correlation between the ratio of pulmonary blood flow to systemic blood flow(QP/QS) and the ratio of RVED/LVED(r=0.44, p<0.025). 3) Thirty three patients were divided into 3 groups according to the pattern of intervent ricular septal motion.(Group "M": Patients with normal septal motion, Group "B": Patients with paradoxical type B septal motion, Group "A": Patients with paradoxical type A septal motion) Preoperatively, the ratio of RVED/LVED was 0.68+/-0.28 in Group "N" (N=9), and 0.88+/-0.23 in Group B (N=9) and 1.14+/-0.34 io Group "A" (N=15). In Group "A, the ratio of RVED/LVED was significantly larger than of Group "N" (p<0.005). Postoperatively, the ratio of RVED/LVED was 0.51+/-0.13 in Group "N" (n=23), and 0.68+/-0.15 in Group "B" (n=7), and 0.79+/-0.14 in Grop "A" (n=3). In Group "A" and Group "B, the ratio of RVED/LVED was significantly larger than that of Group "N" (p<0.005, p<0.01). 4) Postoperative RVED index(18.9+/-4.9) was significantly decreased, compared with preoperative RVED index(27.6+/-8.9) (p<0.005). But there was no significant change in LVED index, before and after operation. 5) Among 10 patients with persistent paradoxical septal motion, repeated echocardiograms were performed in 5 patients. Interventricular septal motion was normalized in 4 out of 5 patients. Thus the ratio of RVED/LVED has significant correlation with QP/QS and the pattern of interventricular septal motion. These results suggest the evidence that interventricular septal motion is determined by septal position at enddiastiole.