A 37-year-old female, who had undergone mitral valve replacement with a monoleaflet mechanical prosthesis for mitral stenosis in 1997, was admitted to outpatient clinic with exertional dyspnea and chest discomfort over preceding 3 months. Her physical examination revealed mildly muffled prosthetic valve sounds, pansystolic murmur on lower left sternal border, and bilateral fine rales at the bases of the lungs. Her electrocardiography showed atrial fibrillation. Transthoracic echocardiography revealed severe left ventricular outflow tract (LVOT) obstruction with mean gradient of 42 mmHg (Fig. 1a, c; Video 1) and severe tricuspid regurgitation. Two-dimensional transesophageal echocardiography (2D-TEE) showed stenosis of mechanical mitral valve (MMV) with pressure half-time (PHT) of 157 ms (PHT was 98 ms in the previous echocardiography) and mean gradient of 13 mmHg. Aortic valve function and leaflet motion of MMV were normal, but there was a hyperechogenic appearance at the ventricular side of the MMV. With the transducer array at 120 , 2D-TEE revealed that the hyperechogenic mass on the ventricular side of the MMV caused narrowing of the LVOT (Fig. 2a; Video 2). Real-time three-dimensional transesophageal echocardiography (RT-3D-TEE) clearly delineated an eccentric semicircular pannus overgrowth which was located anteriorly on the ventricular side of the MMV (Fig. 2b; Video 3), resulting in LVOT obstruction (Fig. 2c; Video 4). Redo mitral valve replacement with a bileaflet mechanical prosthesis (no. 29, St. Jude Medical) and concomitant repair of the tricuspid valve were performed, and anteriorly eccentric semicircular pannus tissue was removed from the left ventricular side of the mitral prosthesis (Fig. 2d). Postoperative transthoracic echocardiography (TTE) revealed normal functioning mitral prosthetic valve with normal LVOT gradients (Fig. 1b, d; Video 5). Prosthetic heart valve obstruction caused by pannus formation is an uncommon but serious complication. Determination of the precise etiology of valvular obstruction is essential, as this can have an impact on the course of surgical versus thrombolytic therapy [1]. Although 2DTEE is the first method of choice in the diagnostic algorithm, exact visualization of pannus is almost impossible [2]. However, RT-3D-TEE is a very useful imaging technique and permits differentiation of pannus overgrowth from other MMV pathologies [3, 4]. Herein, we present an anteriorly localized eccentric pannus overgrowth on the left ventricular side of MMV, which is an uncommon cause of LVOT obstruction.