The ECGs of a 100 consecutive children who had surgical repair of their ventricular septal defects (VSDs) were analyzed for postoperative right bundle branch block (RBBB). Seventy of them had an atriotomy and the other 23 also a ventriculotomy. The ventriculotomy always consisted of a transverse incision a short distance below the pulmonary annulus. Of these children 93 had a perimembranous VSD and the other 7 a pure muscular defect. The ECG results of the 93 children with perimembranous VSDs were statistically analysed. The incidence of postoperative complete RBBB (CRBBB) in the ventriculotomy group was not higher than in the atriotimy group. Infants operated in the first half year of life were more prone to the development of CRBBB than the older children, probably because the VSDs were relatively larger in the younger than the older children. The risk of postoperative CRBBB was less in the children who had direct suture closure of the VSD compared with those who needed a Dacron patch to close the defect. The data in the literature generally indicate a higher incidence of CRBBB after a ventriculotomy than an atriotomy. The absence of this difference and the lower incidence of CRBBB after a ventriculotomy in our series compared with those of several other authors are suggested to be due to the type of ventriculotomy.