A 35 year old female presented with a stroke, all her imaging and biochemical investigations looking for a cause of her stroke was unremarkable apart from interatrial septal aneurysm. Atrial septal aneurysm (ASA) is defined as redundant and mobile interatrial septal tissue in the region of the fossa ovalis with phasic excursion of at least 10 mm during the cardiac cycle (Silver and Dorsey, 1978). Two mechanisms have been proposed to explain the association between (ASA) and cryptogenic stroke: Paradoxical embolism may occur via the patent foramen ovale or that fibrin-platelet particles adhere to the left atrial side of the aneurysm and are dislodged by oscillations of the aneurysm, causing systemic embolism. Diagnosis of (ASA) can sometimes be established by 2D-transthoracic echocardiography, but Trans esophageal echocardiography is more sensitive, in a review of 195 cases (Mugge et al., 1995), 47% were missed with transthoracic echocardiography (Fig. 1). Figure 1 Treatment is controversial with current evidence status (Albers et al., 2008) a reasonable approach is to start antiplatelets to prevent adherence of fibrin to the sepal wall. Anti coagulation is recommended if there is evidence of deep venous thrombosis with or without pulmonary embolism and if the stroke recurs then surgical excision can be considered.