A 20-month-old boy was referred to the cardiac catheterization laboratory for closure of a large prosthetic mitral valve paravalvular leak. At birth, the boy had an unbalanced atrioventricular septal defect with borderline hypoplasia of the mitral component of the common atrioventricular valve and the left ventricle. At the age of 3 months, the boy underwent complete (two-ventricle) repair. The intraoperative findings included deficient left-sided atrioventricular valve tissue. As a result of the surgical septation, the coronary sinus ostium was located in the left atrium. The boy initially did well, but 6 weeks after his initial surgery, progressive and severe mitral regurgitation developed, necessitating mitral valve replacement with a 19-mm St. Jude prosthetic valve in the supraannular position. A small (posteromedial) paravalvular prosthetic mitral leak was noted approximately 6 months after surgery. This leak became progressively worse, resulting in moderate to severe functional mitral regurgitation and worsening left ventricular dilation. A dilated structure posterior to the left atrioventricular groove also was noted to be expanding. This structure was interpreted by echocardiography as an aneurysm of the coronary sinus. Three-dimensional (3D) echocardiography helped to define the anatomy of the leak (Fig. 1a/Movie 1a in supplementry material). At the age of 20 months, the boy was referred for cardiac catheterization with planned closure of the leak.