Echocardiograms were obtained from 48 Japanese children with ventricular septal defect (16 having aortic cusp prolapse, Group I, and 32 without it, Group II). In the case of right coronary cusp prolapse, the right coronary sinus protrudes anteriorly into the right ventricular outflow tract, and thus, the anteroposterior diameter of the aortic root increases. In the case of non-coronary cusp prolapse, the non-coronary sinus bulges posteriorly into the right ventricle, and thus, the aortic root increases in size. For evaluating the degree of these prolapses quantitatively, we measured the aortic root diameter echocardiographically and expressed them as a percent of a normal one. In Group I the aortic root diameter was 131 +/- 9% (mean +/- SD) and in Group II it was 105 +/- 7%, and the difference between the 2 groups was statistically significant (p less than 0.001). In Group I 14 of the 16 patients had a value greater than 120%, while all 32 patients of Group II had a value smaller than 120%. Thus, in children with ventricular septal defect, an aortic root diameter greater than 120% of normal suggests the presence of aortic cusp prolapse. Systolic semiclosure of the aortic valve was found in 8 patients of Group I (50.0%) and in 2 of Group II (8.7%). Coarse systolic fluttering of the pulmonary valve with an amplitude of greater than 3 mm was detected in 6 of Group I (40.0%) and in 3 of Group II (10.0%). Therefore, semiclosure of the aortic valve and fluttering of the pulmonary valve are considered to be also useful for evaluating aortic cusp prolapse qualitatively.