OBJECTIVE Inferior sinus venosus defect is an unusual form of interatrial communication with few published data on surgical outcomes. We sought to compare outcomes of surgical repair of inferior sinus venosus defect with those of large secundum atrial septal defects. METHODS Patients undergoing surgical closure of an isolated interatrial defect were reviewed, and those with inferior sinus venosus defect were identified on the basis of predetermined anatomic criteria. For each case, 2 controls with secundum atrial septal defect, matched for age and year of surgery, were selected. Technical outcome scores and other perioperative outcomes were compared. RESULTS Compared with the secundum atrial septal defect group (n = 90), the inferior sinus venosus defect group (n = 45) had worse technical outcome scores (P = .02), a higher rate of reintervention (9% vs 1%, P = .04), longer median total cardiopulmonary bypass (48 vs 39 minutes, P < .001) and crossclamp (29 vs 20 minutes, P < .001) times, and were more likely to stay more than 1 day in the intensive care unit (20% vs 8%, P = .04) and more than 3 days in the hospital (29% vs 13%, P = .03). Only 16 (36%) of the patients with inferior sinus venosus defect had a correct diagnosis preoperatively. Patients with an incorrect diagnosis had worse technical outcome scores than the secundum atrial septal defect group (P = .003), whereas those with a correct diagnosis had scores similar to those of the secundum atrial septal defect group (P = .55). CONCLUSIONS Compared with patients with secundum atrial septal defect, patients with inferior sinus venosus defect have more residual defects and longer durations of cardiopulmonary bypass and hospitalization. Rates of misdiagnosis of inferior sinus venosus defect are high and associated with worse technical outcome scores. Accurate preoperative diagnosis of this lesion may lead to improved outcomes.