The anatomic and functional characteristics of 310 consecutive patients undergoing repair of atrioventricular (AV) septal defects were studied. The characteristics of the six leaflets of the common (139 patients) or two-orifice (171 patients) AV valve are described. Interventricular communications were present in 156 patients and major associated cardiac anomalies in 73 (tetralogy of Fallot 20, double-outlet right ventricle nine, and others). Fifty-one patients (16%) died in hospital. The incremental risk of young age disappeared after 1976. Severity of preoperative AV valve incompetence and New York Heart Association (NYHA) functional class affected hospital mortality. Current risks for patients without major associated cardiac anomalies are 0.6% to 4% for patients in NYHA Class III without an interventricular communication, depending on the degree of preoperative AV valve incompetence, and 5% to 13% for such patients with an interventricular communication. Actuarial survival at 12 1/2 years postoperatively is 95% when preoperative AV valve incompetence is absent or mild and 88% when it is more severe (Grade 3, 4, or 5). Actually, 92% of hospital survivors are alive and in NYHA class I or II. Failure of the AV valve repair occurred in 32 (10%) of the 310 patients and occurred more frequently when AV valve incompetence was severe preoperatively. Its frequency is reduced by current repair techniques, except in patients without interventricular communication.