A 46-year-old woman, who was intermittently treated with antibiotics prescribed by four different physicians during two months, was hospitalized because of recurrent fever. Physical examination revealed a pulse rate of 92 beats/min, blood pressure of 140/67 mmHg, and a grade 4/6 diastolic murmur heard loudest at the third left sternal border. The first 2 sets of blood cultures were positive for Streptococcus mutans. However, her laboratory examination, which could be affected by “incomplete” antibiotic treatments, revealed mild inflammation with an elevated C-reactive protein level of 3.3 mg/dL and a white blood cell count of 9130/mm3. A transthoracic echocardiogram showed moderate aortic regurgitation due to the bicuspid aortic valve, and a mildly enlarged left ventricular diastolic dimension (57 mm). In addition, a mass was seen on the medial side of the anterior mitral leaflet (A3), against which the aortic regurgitant blood flow was directed ([Fig. 1a]). An “ellipsoid body-like” mass was clearly observed on a three-dimensional echocardiogram ([Fig. 1b]). Transesophageal echocardiograms showed small vegetations attached to the noncoronary cusp of the aortic valve ([Fig. 2a, b]), and the mass at the mitral valve ([Fig. 2c]). A repeat transesophageal echocardiogram after intravenous administration of penicillin G for 1 month showed no changes in the size and shape of the mass, but an echolucent area in the mass was apparent ([Fig. 2 d]). Antibiotic therapy was continued with oral penicillin for 1 more month. Transthoracic and three-dimensional echocardiographic examinations were repeated, and the mass was found to have transformed into a saccular structure compatible with mitral valve aneurysm ([Fig. 1c, d]). Therefore, we concluded that the mass was an abscess formed by aortic valve endocarditis, and its liquid content was drained during the antibiotic therapy. A 64-row multi-detector computed tomography image with intravenous contrast also detected a saccular structure at the mitral valve ([Fig. 3a]). Surgical findings confirmed aortic valve endocarditis, and the infected valve was replaced with a mechanical valve. Observation of the anterior mitral leaflet revealed an aneurysm with an orifice to the left ventricle, but no perforation ([Fig. 3b]). The mitral valve was repaired using autologous pericardium, and the postoperative course of the patient was satisfactory, but it resulted in an uneventful outcome.