Between 1962 and 1986, 51 patients 50 years of age or older (mean, 59.7 years; range, 50 to 77 years) underwent operative closure of atrial septal defect. The mean pulmonary vascular resistance was 1.7 +/- 0.79 wood units and the mean pulmonary blood flow-systemic blood flow ratio was 2.8 +/- 1.4. Patch closure with pericardium or Dacron was performed in 36 patients (70%), while 15 patients (30%) underwent primary closure. There were no operative deaths, and one patient was not available for follow-up. The remaining 50 patients (98%) have been followed up for 0.5 to 25 years (mean, 9.6 years). Mean preoperative New York Heart Association classification was 2.40 +/- 0.70. This improved significantly to 1.21 +/- 0.42 at follow-up, with all patients improving by at least one category. No patient received long-term anticoagulation treatment, and no pulmonary or systemic emboli were identified. Two (13%) of 15 patients who underwent primary closure developed septal dehiscence. Actuarial survival was 93%, 86%, and 79% at 5, 10, and 15 years, respectively. Atrial septal defect closure can be safely performed in older patients with excellent results, provided the ratio of systemic to pulmonary blood flow is greater than or equal to 1.5 to 1.0, the pulmonary vascular resistance is low, and the shunt remains left to right. Primary closure should be discouraged, and postoperative anticoagulation therapy appears unwarranted.