OBJECTIVE Ostium primum atrial septal defect (ASD I) is very rarely observed in the adults. Although ASD I was first corrected surgically almost 50 years ago, the efficacy of surgical treatment in adults has not been well documented. Thus, the long-term outcome of patients aged over 20 years and having surgical repair of ASD I was examined. METHODS Fifteen patients (10 males, five females; mean age 31.4+/-13.1 years; range: 20 to 56 years) who had surgical repair of ASD I between 1982 and 2000 were followed. All patients were examined physically and underwent chest radiography, ECG and echocardiography (cross-sectional and Doppler) before and after surgery. Autologous pericardium was used to close the defect in 14 patients, and a direct suture was used in one patient. Mitral valvuloplasty (repair of the cleft) was performed in 12 patients, and De Vega annuloplasty in eight. RESULTS There were no hospital deaths. In one case a pacemaker was implanted five days after surgery because of complete heart block. Preoperatively, nine patients (60%) were in NYHA classes III and IV; at the end of follow up, 14 (93.3%) were in classes I and II. Preoperatively, 13 patients had sinus rhythm, and one each had atrial fibrillation (AF) and rhythm from the atrioventricular sinus. During follow up, three patients developed AF which was treated successfully with electrical cardioversion. The preoperative mean cardiac volume index of 695 +/- 216 ml/m(2) was reduced significantly after repair to 523 +/- 108 ml/m(2) (p < 0.05). Before surgery, mitral regurgitation was observed (severe in five cases, moderate in seven, mild in three). Postoperatively, a residual intracardiac shunt was identified in one case. Postoperative mitral regurgitation was noted in six patients (moderate in two, mild in four). The right ventricular dimension was decreased significantly, from 5.0 +/- 1.5 mm before surgery to 3.2 +/- 0.6 mm after repair (p < 0.001). CONCLUSIONS Patients aged over 20 years with ASD I benefit from surgical repair of the defect.