Secundum type atrial septal defects were closed in 32 adults aged 30 to 57 years, with the use of cardiopulmonary bypass. Before operation, 12 had functional class II disease and 11 class III disease (New York Heart Association classification). Eight had a history of congestive cardiac failure, and 6 had atrial fibrillation. Cardiothoracic ratio ranged between 50 and 76 percent. Sixteen patients had moderate to severe pulmonary hypertension, but none had reversed shunt. There was 1 operative and 1 late death. Neither death was related to the complications usually associated with atrial septal defect in this age group. The remaining patients were followed up for 6 months to 12 years. All but 2 had functional class I disease after operation. Postoperatively a statistically significant reduction was noted in cardiothoracic ratio and pulmonary arterial pressures, and there was consistent rise in calculated pulmonary to systemic resistance ratio. A small residual shunt of 1.6:1 was present in 2 patients. It is concluded that irrespective of preoperative complications (pulmonary hypertension, atrial fibrillation and congestive heart failure), repair of atrial septal defect with left to right shunt in adults produces considerable clinical and hemodynamic improvement, and has low mortality and morbidity rates.