Twelve adult patients with ostium primum atrial septal defects (incomplete endocardial cushion defect) who underwent surgical repair of their lesions were evaluated in the late postoperative period. All had closure of the low-lying atrial septal defect, with suturing of the mitral valve cleft in 11 patients. Although the patients benefited symptomatically from the surgery, all had residual cardiac murmurs. Postoperative cardiac catheterization and left ventriculography in eight revealed successful closure of the atrial septla defect, but three demonstrated residual mitral insufficiency. In spite of the successful surgical repair in these patients, bacterial indocarditis prophylaxis should be continued in view of the residual murmurs and valvular abnormalities.