We have previously reported isolated cases of anatomical and functional closure of ventricular septal defects in tricuspid atresia. To study this phenomenon further, clinical, angiographic, and pathological findings in 20 consecutive cases of tricuspid atresia were reviewed. Sixteen cases were found to have normally related great arteries (type I) and 4 had transposition (type II). In 6 of these patients there was evidence of closure of a ventricular septal defect; in 3 this was complete and in the other 3, partial. Five of these ventricular septal defect closures occurred in type I patients and one among type II. Increasing cyanosis and polycythaemia and/or disappearance of a previously heard murmur were observed in all patients; these signs are more conspicuous in complete closure than in partial. The incidence of closure of ventricular septal defect in tricuspid atresia is 38 per cent and approximates to the incidence of spontaneous closure of isolated ventricular septal defects. Progressive muscular "encroachment" of the margins of ventricular septal defects with subsequent fibrosis and covering by endocardial proliferation is the most likely mechanism of closure in tricuspid atresia. The factors initiating the closure of ventricular septal defect remain unknown, but because of its occurrence in patients both with and without previous shunt operations, it is reasonable to assume that closure of ventricular septal defect in tricuspid atresia is not initiated or accelerated by these surgical shunts. Recent developments in surgical technique permit total surgical correction of tricuspid atresia at least in a physiological sense, but these operations can be successfully performed only in older children (over 8 years). Thus, palliation is essential in younger patients until they reach the age for total correction. A Blalock-Taussig shunt is recommended in preference to a Glenn anastomosis because the latter may leave the left pulmonary circuit without blood supply if the ventricular septal defect closes. In type II cases, a large and non-restrictive ventricular septal defect is essential for survival of the patient after the Fontan operation. For this reason, the size of the ventricular septal defect should be evaluated before and at the time of surgical correction. If the ventricular septal defect is small in a type II case, resection of the ventricular septum or a complete bypass of the ventricular septal defect and right ventricle is necessary at the time of the Fontan operation.