A two-dimensional echocardiographic study was performed in patients (pts) with isolated ventricular septal defect (VSD). Group I included 48 pts with membranous VSD (aged from 3 months to 11 years) diagnosed by cardiac catheterization and angiocardiography. In 25 of 27 pts demonstrated a defect only by two-dimensional echocardiography (2DE), mean pulmonary artery pressure (MPAP) was higher than 25 mmHg and 26 of these 27 pts showed a large left to right shunt of more than 30 per cent, and the remaining one pt had Eisenmenger complex. In 15 of 16 pts demonstrated VSD with the pouch formation of the septal leaflet (PSL) by 2DE, MPAP was lower than 25 mmHg. These 16 pts showed various left to right shunt ratios (small to large). In all 4 pts demonstrated VSD with membranous septal aneurysm (MSA) and 1 pt demonstrated no findings by 2DE, MPAP was low (under 25 mmHg) and a left to right shunt was small. Group II included 400 pts with membranous VSD (aged 11 days to 15 years) diagnosed clinically. All 221 pts demonstrated VSD had PSL or MSA. PSL was demonstrated in 261 pts, MSA in 27 pts, both PSL and MSA in 33 pts and no findings in 79 pts by 2DE. Group III included 41 pts with spontaneous closed VSD (aged 3 months to 8 years) diagnosed clinically. PSL was demonstrated in 15 pts, MSA in 11 pts and no findings in 15 pts by 2DE. On the basis of these observation, PSL and MSA may play an important role in spontaneous diminution or closure of membranous VSD, and in most of VSD pts without PSL and MSA pulmonary hypertension is shown and spontaneous diminution of VSD size can not be anticipated. Visualization of PSL and MSA by 2DE may predict the prognosis and is clinically very useful in the management of pts with isolated membranous VSD.