Congenital aneurysm of the left atrial appendage is a rare anomaly, usually presenting in adult life. The case reported is that of a 55-year-old man who died of cerebral embolism originating from a thrombus in a congenital aneurysm of the left atrial appendage. The cardiac silhouette 11 years previously had suggested a cardiac tumour or a pericardial cyst. As judged frm 14 published cases, the major manifestations of these aneurysms are an abnormal cardiac silhouette in the x-ray, supraventricular tachycardia, and systemic embolism. Angiocardiography appears to be the method of choice in establishing the diagnosis. Resection of th aneurysm is the recommended form of treatment.