An 81-year-old woman, who had undergone mitral valve replacement (MVR) with a porcine bioprosthesis after mitral valve repair, presented with hemolysis 4 years and 6 months after MVR. Transthoracic echocardiography (TTE) revealed trivial mitral regurgitation, which was diagnosed based on the observed perivalvular leakage. Hemolysis gradually increased, and she developed dyspnea and edema 2 years after the appearance of mitral regurgitation. We performed a reoperation. Intraoperative transesophageal echocardiography (TEE) after intubation showed no perivalvular leakage of the mitral prosthesis, but transvalvular leakage through a leaflet perforation was present. The leaflets of the bioprosthesis had slit-shaped perforations at their hinges. There was no sign of infection on the leaflet or annulus. We implanted a new bioprosthesis after removal of the deteriorated valve. The postoperative course was uneventful. Microscopic examination verified collagen degeneration, histiocyte infiltration, and hyalinization. It is important to perform TEE to rule out structural valve deterioration (SVD) even when regurgitation occurs soon after valve replacement.