The clinical course of 109 patients who underwent closure of a sinus venosus atrial septal defect is reviewed, with emphasis on the incidence, type, and severity of arrhythmias before and after operation. There were no operative deaths and only 1 late death. No instances of obstruction of the superior vena cava were detected clinically. One patient had a probable hemorrhagic pulmonary infarction that cleared; another may have a persistent left-to-right shunt. Excellent results were achieved in 72% of the patients, good results in 5%, and poor results in 3%. The type of arrhythmia, both before and after operation, varied with the age of the patient: younger patients had bradyarrhythmias, and older patients had tachyarrhythmias. Ten patients experienced persistent new arrhythmias postoperatively, but none were disabled, required a pacemaker, or died. The arrhythmias in all severely symptomatic patients were supraventricular tachycardias that had occurred before operation. Although functional classification after operation was clearly related to age at the time of the procedure (with younger patients having the best functional results), 19 of 21 patients over age 40 were noticeably improved after surgical closure of the sinus venosus atrial septal defect.