We studied 12 patients (6 men and 6 women) with a mean age of 47 years with stroke (n = 11) or transient ischemic attack (n = 1) in relation with an atrial septal aneurysm. Ten patients showed also a patent foramen ovale. Infarcts involved the posterior cerebral artery (n = 3), superficial middle cerebral artery (n = 2), deep middle cerebral artery (n = 3), deep and superficial middle cerebral artery (n = 1), posterior inferior cerebellar artery (n = 1) and posterior inferior cerebellar and superficial middle cerebral artery (n = 1) territories. Embolism was likely in all patients, although coexisting small-artery disease could not be excluded in 2 patients. The exact mechanism remained unsettled, although paradoxical embolization of thrombi formed within the aneurysm through the interatrial wall should be considered. Our study emphasizes the importance of detecting potential cardiac sources of embolism by routine transthoracic and transesophageal echocardiography in stroke of unknown origin.