Right ventricular endomyocardial biopsies were performed in nine patients with mitral valve prolapse (MVP), and the histological features were compared with the clinical findings. All of them had atypical anterior chest pain and/or dangerous ventricular arrhythmias, but the grade of MVP was mild. Five patients had non-specific ST-T changes on resting 12 leads ECG, four had ischemic ST depression on treadmill exercise ECG, and five had dangerous arrhythmias on 24 hour Holter monitoring ECG. Resting echocardiography showed normal left ventricular function in all, but exercise echocardiography revealed reduced increment of % fractional shortening (delta % FS) in five patients. Right ventricular endomyocardial biopsy findings disclosed endocardial thickening and interstitial myocardial fibrosis in eight patients, myocardial hypertrophy in two, myocardial degeneration in five, and myocardial disarray in five, although these changes were mild. These results suggest that in patients with MVP, there is a subgroup with endomyocardial abnormalities, and these might have some relations to the variety of clinical findings in MVP.