Atrial septal defect (ASD) is one of the simplest cases among the surgically correctable congenital cardiac diseases. Easiness of the correction usually causes underestimation of the pathology. Angiography as well as echocardiography might be misleading in some cases. The preliminary diagnosis of the presented case was secundum ASD. During operation an unusual type of partial anomalous pulmonary venous connection (PAPVC) was noticed. Pulmonary veins of the entire left lung were draining to the right atrium directly. There was no other coexisting cardiac pathology including atrial septal defect. To our knowledge, there is no similar case in the literature. ASD is a well known congenital cardiac disease. Coexisting anomalies increase the surgical importance of the defect. Cardiologists should suspect and search for even the rarest pathologies during preoperative evaluation to prevent diagnostic misinterpretations and surgical surprises. Trying to explore the relations of cardiac chambers and define the pathology during extracorporeal circulation is time consuming. Despite this, a routine and meticulous exploration should not be neglected to prevent unexpected results.