Abstract Cardiac decompensation has been reported in a rather small percentage of all cases of arteriovenous aneurysm. Even when there is severe decompensation, the electrocardiogram often shows few changes. Published electrocardiograms showing marked abnormalities are very rare. A case is presented showing pronounced abnormalities in the electrocardiogram. Tracings were made prior to surgical cure of the aneurysm and after the operation, over a period of 123 days. They showed a remarkably slow return to normal, though clinical improvement was rapid. Auricular fibrillation in this case did not cease spontaneously after operation but ceased with quinidine therapy, and showed no inclination to return. Other cases on record having decidedly abnormal electrocardiograms are compared with the present case. Certain clinical features of this case are compared with other cases from the literature.