A continuous, machinery-like murmur in the left upper anterior portion of the chest near the sternum that occurs without history of trauma is so characteristic of patent ductus arteriosus that one may neglect to consider other lesions that produce a similar murmur in this location. We present a case of arteriovenous fistula, presumably congenital in origin, that simulated patent ductus arteriosus but was accurately diagnosed and localized by means of venous catheterization and angiocardiography. REPORT OF A CASE A 31-year-old woman was referred to the cardiovascular service for study because of a continuous, machinery-like murmur over the upper part of the sternum and in the first left costal interspace. The patient had been semicomatose owing to severe diabetic acidosis when admitted to the medical service on Feb. 4, 1953. On admission, the blood sugar level was 471 mg. per 100 cc.; serum carbon dioxide content, 15 vol. %; serum potassium level,