Among 61 heart specimens of tetralogy of Fallot with or without pulmonary atresia, four presented with an accessory tricuspid valve leaflet. This structure caused partial or almost complete obstruction of the ventricular septal defect. Depending on the morphology, the accessory tissue was classified into "mobile" or "fixed" types. The "mobile" variety was tethered by long chordae tendineae which permitted a wide excursion of the leaflet through the ventricular septal defect into the left ventricular outflow tract where it represented a potential cause of obstruction. The "fixed" variety was attached to the edges of the defect by short chordae which reduced considerably its movements. This type created a fixed obstruction of the ventricular septal defect without involving the subaortic left ventricular outflow tract. The precise morphology of the accessory tricuspid valve tissue is of considerable surgical significance. When mobile, the tissue must be resected at the time of surgical repair. When fixed it can be used as a suture anchorage during closure of the ventricular septal defect.