We performed tricuspid annuloplasty aggressively in association with mitral and combined mitral and aortic valve disease. Because tricuspid regurgitation (TR) may produce significant morbidity if not corrected. From March 1986 to September 1990, for the purpose of studying the clinicopathology of the biopsied right ventricular myocardium in 54 patients with mitral valvular disease, the diameter of right ventricular myocardial cells and diffuse interstitial fibrosis of biopsied myocardium were measured quantitatively. The degree of diffuse interstitial fibrosis was assessed by the point-counting method, and mean percentage fibrosis (% fibrosis) was noted as 16.1%, % fibrosis was correlated with the severity of TR (r=0.36). Mean diameter of right ventricular myocardial cells was 19.3μm in size. The diameter of right ventricular myocardial cell was correlated with pulmonary arterial pressure (r=0.51). More aggressive narrowing of tricuspid valve ring using DeVega's method was performed on 39 of 54 patients. The evaluation of TR was done by Doppler echo. Postoperative residual TR was observed in four patients of total postoperative patients within mean follow-up period of 41.0 months. In the 4 patients, % fibrosis were in high % fibrosis (>20%). In comparison to the group with low % fibrosis (<15%) (n=27), the group with high % fibrosis (>20%) (n=9) was more serious in severity of TR by Doppler echo (2.4 vs. 1.7), age (58.2 vs. 51.6), NYHA classification (3.2 vs. 2.6) before surgery. In conclusion, open right ventricular myocardial biopsy findings suggest that the degree of myocardial fibrosis is correlated with severity of tricuspid regurgitation. And the patients whose % fibrosis are more than 20% are presumed poor right ventricular function. The diameter of right ventricular myocardial cell is correlated with pulmonary arterial pressure, that is, right ventricular hypertrophy is associated with pulmonary hypertension.