An unusual case of an interatrial communication through a defect of the coronary sinus together with coarctation of the aorta is reported. In spite of two cardiac catheterization studies, this diagnosis was missed by two different cardiology departments, and anomalous pulmonary vein connection was suspected instead. This illustrates the preoperative diagnostic difficulties which continue during surgery, when exploration of the right atrium fails to detect the suspected atrial defect. This rare anomaly should be considered in the differential diagnosis of an atrial left-to-right shunt, when the largest "oxygen step-up" is at the level of the lower right atrium, and no evidence of an atrial septal defect or transposition of the pulmonary veins can be found.