We studied possible factors to cause left ventricular rupture after mitral valve replacement and the prevention in eight patients of 1, 046 receiving mitral valve replacement between September, 1965 and August, 1991. The age at operation ranged from 43 to 67 years old (average 58 years old), and there were one man and seven women. According to the Treasure and Miller's classification, the type of rupture was type I in 5, type II in 2, and type III in 3. The onset time of rupture was immediately after cardiopulmonary bypass in 3 and at the time of chest closure in one. In four patients it occurred 11 hours, 14 hours, 18 hours and 25 hours after operation, respectively. In 8 patients, repair was performed with external closure under heart beating and in five patients with both internal and external closure during cardiac arrest under cardiopulmonary bypass. Two patients under cardiopulmonary bypass were successful for hemostasis, however, they died with low cardiac output syndrome, following to multiple organ failure 2 or 44 days after operation, respectively. The repair was not successful in 6 patients. In four patients the left ventricular rupture occurred immediately after hypertension and pathological findings showed severe myocardial degeneration of left ventricular muscle in all of them. These findings may suggest that hypertension after the operation is one of major factors to cause left ventricular rupture and thus the careful management of the systemic blood pressure after mitral valve replacement is effective to prevent the left ventricular rupture.