What congenital abnormality would result in eccentric mitral regurgitation, stenosis, and a poorly visualized posterior mitral leaflet? A woman in her 70s with pulmonary hypertension, restrictive lung disease from severe kyphoscoliosis, and heart failure with preserved ejection fraction presented with worsening dyspnea despite stable pulmonary function test results. On echocardiography, there was bulky calcification of the anterior mitral leaflet with poor visualization of the posterior leaflet (Figure, A). Severe mitral stenosis was found (mean transvalvular gradient, 16 mm Hg; estimated orifice size, 0.8 cm2) with eccentric posterior regurgitation of moderate severity. Computed tomography revealed elongation and thickening of the anterior mitral leaflet and absence of the posterior leaflet (Figure, B), leading to the diagnosis of unileaflet mitral valve. Given that the patient was not a candidate for surgical or percutaneous repair, medical therapy was pursued, with symptomatic improvement at follow-up.