Abstract Aneurysms involving the fibrous body of the atrioventricular ring are distinctly different from other types of left ventricular aneurysm. They have their orifices in the atrioventricular fibrous body and generally track along the atrioventricular ring or into the superior ventricular septum. These aneurysms are of uncertain etiology, although a structural defect of the atrioventricular ring has been suggested. At least one case is clearly related to trauma. We are reporting a “fibrous body aneurysm” which occurred in a patient with subacute bacterial endocarditis involving the aortic valve. Brucella suis was the causative organism. The relation of the orifice of the aneurysm to the affected aortic valve strongly suggests that it was mycotic in origin.