Hypoplasia of the mitral leaflets is rarely reported in adults. We reported a 45-year-old man presented to us with palpitation, dyspnea, and severe mitral regurgitation. In echocardiographic examination, an immobile hypoplastic posterior mitral leaflet (PML) was detected. A review of clinical and echocardiographic findings of all reported cases revealed that mitral regurgitation was the most common finding associated with PML hypoplasia/uni-leaflet mitral valve. PML hypoplasia is rare and often an incidental diagnosis during echocardiography, which could be relatively well tolerated since adulthood and carries a good prognosis.