A 18-year-old boy presented for cardiological evaluation for a previous pleuritic chest pain. Physical exam was normal and ECG showed an early repolarization pattern. Transthoracic echocardiogram revealed an ondulating structure on the left side of the interatrial septum (IAS), without other abnormalities. Transoesophageal study was performed in order to define better the IAS anatomy and assess for other anomalies. It showed a high mobile membrane adjacent and parallel to the IAS with flow into its chamber. Intravenous agitated saline contrast injection excluded the presence of patent foramen ovale (PFO) or interatrial septum defect. We review literature about atrial septal malformations.