Atrial septal aneurysm is an uncommon occult cardiac source of cerebral embolism. It is usually asymptomatic, and clinical cardiologic examination and electrocardiography fail to reveal its presence. We report a case of a 34-year-old woman with sudden right hemiplegia and aphasia from occlusion of the left carotid siphon in whom an atrial septal aneurysm was detected by two-dimensional echocardiography. The absence of atherosclerotic risk factors and vascular lesions proximal to the carotid occlusion strengthened a causal relation between atrial septal aneurysm and cerebral infarction. Consequently, two-dimensional echocardiography may be advisable in every patient with unexplained ischemic stroke to detect possible occult embolic cardiac abnormalities.