We report a case of mitral valve aneurysm in a 30-year-old man presenting with fatigue, malaise, and fever 10 weeks after emergent aortic valve replacement for endocarditis. The transthoracic echocardiogram demonstrated perivalvular aortic regurgitation, but no abscess cavity was defined. Cardiac magnetic resonance imaging (MRI) revealed a mitral valve leaflet aneurysm. Both aortic and mitral valves were replaced with mechanical prostheses. Pathology of the excised mitral valve showed a focally hemorrhagic aneurysm of the anterior leaflet with myxoid degeneration and focal calcification. Early diagnosis and intervention are important to treat this rare, potentially fatal complication of aortic valve endocarditis. Cardiac MRI provided an accurate and useful preoperative diagnostic evaluation.