We report a case of a newborn with a prenatal diagnosis of hypoplastic left heart syndrome (HLHS), referred at 37 weeks of gestation. The fetal echocardiogram had shown classic morphologic aspects of HLHS with patent mitral valve and aortic valve atresia. The atrial septum was thickened and the foramen ovale (FO) appeared to be patent. It was a normal full-term delivery, with birth weight of 2540 g. Orotracheal intubation and mechanical ventilation were immediately necessary, as well as intravenous prostaglandin, due to clinical deterioration. Transthoracic echocardiography performed in the first hour of life confirmed the diagnosis, but doubts were raised about the patency of the FO. Cardiac catheterization was performed and Rashkind balloon atrial septostomy was attempted, without success. The baby died and pathologic examination revealed HLHS with intact atrial septum, dilated coronary sinus and pulmonary lymphangiectasia. In this case, the dilated coronary sinus led to a mistaken prenatal diagnosis of FO patency with left-to-right shunt.