Mitral and aortic valve regurgitation is commonly found in osteogenesis imperfecta (OI) patients, however, little is known about the myocardial involvement in this disorder. An 82-year-old man with OI developed heart failure and was admitted to our hospital. Echocardiogram revealed severe mitral regurgitation without left ventricular (LV) dilatation, but with LV wall thickening. Histological analysis exhibited interstitial fibrosis of the myocardium in addition to myxoid changes of the mitral leaflet. These findings suggest that OI patients may develop LV remodeling together with diastolic dysfunction.