Background: Unroofed coronary sinus syndrome (UCSS) is rare and often associated with the left superior vena cava (LSVC). We report our experience of the clinical features, diagnosis, associated anomalies, surgical procedures, and late outcomes of UCSS during a 20-year period. Methods: One hundred and fifty-nine patients with UCSS were treated surgically and followed up between May 1998 and May 2019. UCSS was confirmed by preoperative echocardiography or computerized tomography (CT) scan in 97 patients and by the surgeons intraoperatively in 62 cases, and 100 cases were associated with LSVC. Among these patients, eight cases underwent ligation, and the intracardiac tunnel or baffle was reconstructed in 71 cases to ensure drainage of the LSVC into the right atrium (RA), and the extracardiac procedure was to lead the LSVC into the RA in three patients. Coronary sinus opening was diverted to the RA by the atrial septal patch in three cases. Thirteen patients underwent the reroof procedures, and two patients had untreated LSVC. The associated cardiac lesions were corrected concomitantly. Results: There were five in-hospital deaths. Follow-up of 143 patients of early survivors was done, and no deaths were reported. No serious complications were observed, with the exception of avulsion of the internal tunnel patch in one patient. Conclusion: UCSS is often misdiagnosed during preoperative evaluation of congenital heart diseases. Preoperative transthoracic echocardiography remains the most important method in the diagnosis of UCSS. We adopted multiple surgical approaches to manage the different types of UCSS with LSVC with good results. K E Y W O R D S cardiac surgical procedures, congenital heart disease, echocardiography, left superior vena cava, unroofed coronary sinus 1 | INTRODUCTION Unroofed coronary sinus syndrome (UCSS) is a group of rare, complex cardiac malformations, which refers to the partial or complete defect of the coronary sinus (CS) septum, which leads to a direct communication between the CS and the left atrium (LA). UCSS is often associated with the left superior vena cava (LSVC; 80%-90%), with an overall incidence of approximately 0.3%-0.5%, and that in congenital heart diseases of approximately 3%-10%. 1 Preoperative diagnosis of UCSS was considered challenging due to the lack of specific clinical manifestations and examination methods. We treated 68,740 patients with congenital heart diseases from May 1998 to