The authors report the case of a 40 year old woman who had an ischemic stroke. The initial investigation including a complete blood clotting analysis, failed to demonstrate the cause. Five years later, the investigations were completed systematically by transesophageal echocardiography which demonstrated an isolated thrombus localised in the left atrial appendage though the localisation heart was in sinus rhythm and morphologically normal. This case illustrates in the diagnostic yield of transesophageal echocardiography in the investigation of systemic embolism in young patients who have had a cerebral ischemic event of unknown cause, even in the absence of predisposing cardiac cause.