A 15-year-old girl was admitted for repair of partially unroofed coronary sinus syndrome. A preoperative echocardiographic study disclosed the dilated coronary sinus and a draining blood flow into the right atrium from both the left atrium and the end of the coronary sinus. At surgery, the distal portion of the coronary sinus roof was found to be absent. The defect was repaired through the coronary sinus orifice with a patch. No electrophysiological conduction abnormalities developed and follow-up echocardiography showed neither residual shunts nor coronary sinus blood flow disturbances.