Sinus venosus atrial septal defect (SVD) is a rare cardiac abnormality in adults. Particularly, the inferior type is difficult to depict by transthoracic echocardiography because of its infero-posterior location to the fossa ovalis. We describe the case of a 33-year-old woman whose chest X-ray taken during bronchopneumonia revealed a cardiomegaly. Further echocardiographic investigations showed an underlying inferior SVD, an incomplete cor triatriatum dexter and a large patent foramen ovale (PFO). The diagnosis was confirmed by cardiac magnetic resonance imaging (CMR) and during surgical repair.