1. Mitral valve replacement with Starr-Edwards' ball-valve prosthesis was performed in 65 patients with mitral insufficiency or stenosis from September 1965 to May 1971 at the University Hospital of Niigata and its affiliated hospitals. 2. Functional tricuspid insufficiency was associated in 30 cases. According to its severity, tricuspid annuloplasty was undertaken in 7 patients and tricuspid valve replacement in 7 patients. 3. Hospital mortality was 4.6 per cent (3 deaths) and late mortality was 3.1 per cent (2 deaths). Cerebral embolism was the most hazardous complication through the entire period and two patients were in severely disabled condition. 4. Some considerations concerning to surgical results, especially of cases associated with tricuspid insufficiency are described. In addition to the palpation findings during surgery pulmonary artery and right atrial pressures obtained at the preoperative cardiac catheterization should be used in order to indicate the surgical intervention for tricuspid insufficiency.