Experience inthesurgical treatment ofendocardial cushion defects hasbeenreported bya number ofauthors (Cooley, Kirklin, andHarshbarger, 1957;McGoon,DuShane, andKirklin, 1959; Crafoord and Senning, 1960;Dubostand Blondeau, 1960;Ellis, McGoon,andKirklin, 1960;Gerbode, Johnston, Robinson, Harkins, and Osborn, 1961;Maloney, Marable, andMulder, 1962;Scott, Hauck,Nadas, andGross, 1962). Thesepapershavefocused attention on the technical details ofsurgical repair, andforthe mostparttheyhavetakensurvival as their standard ofa successful operative repair. In evaluating theexperience at the Montreal Children's Hospital, webecameawarethatdegrees ofcorrection arepossible, andwiththis inmind theresults inthe27patients withendocardial cushion defects whohavebeensurgically treated werecritically reviewed. Eighteen ofthese hadan ostiumprimumatrial septal defect without an interventricular communication andthey formthe basis ofthis report. The18patients wereoperated uponfromfive years tofourmonths ago.Atthetimeofoperation there wasacomplete cleft oftheaortic leaflet of themitral valve in15patients, a partial cleft in onepatient, andintwopatients nocleft wasfound. Onepatient hadnoatrial septumwhatsoever, there being asingle commonatrial chamberintowhich drained twosuperior venaecavae, theinferior venacava,andthefourpulmonary veins. In another patient, themitral valve wasunusual in thatthere wascentral fusion between theaortic andthemuralleaflets, producing abridge ofvalve tissue extending between thetwoleaflets. These abnormalities areshowninFigure 1.Inthesurgicalrepair themitral valve cleft wasclosed with interrupted sutures. Themitral insufficiency was thentested and iffurther suturesappeared indicated, they wereadded.Abnormalchordae inserting directly intotheventricular septum were searched forbutwerenotrecognized despite our realization thattheyareoften present asreported by Edwards(1960). The upperpartof the ventricular septumimmediately belowtheatrioventricular valveswas probedin orderto determine whether asmall ventricular septal defect mightbepresent. Thelowatrial septal defect was closed inallbutoneofthepatients withapatch ofteflon felt sutured inplace withmultiple peripheral sutures. Inferiorly, thepatchwassutured tovalve tissue. All18patients survived operation. Withthe exception ofthelast patient, whowasoperated upontoorecently forevaluation, theyhavebeen evaluated uptofouryears post-operatively, with