We conducted mitral valve replacement (MVR) in a patient with mitral regurgitation (MR) complicated with idiopathic thrombocytopenic purpura (ITP). The patient was a 62-year-old male who was diagnosed to have grade IV MR. However, a decrease in platelet count was noted (5.6 x 10(4)/microliter) when he has admitted to this hospital for operation. After detailed examination, he was diagnosed to have ITP. Though mass intravenous infusion of gamma-globulin (400 mg/kg/day, for 5 days) was done before the operation, the treatment was not successful and splenectomy was consequently conducted. In view of the influence of invasive splenectomy, the change in platelet count was carefully observed thereafter. The count subsequently increased to reach a peak (26.9 x 10(4)/microliter) after 2 weeks from the splenectomy. MVR was conducted when the count started to decrease again after the peak. The operation was safely completed without any complication such as hemorrhage during and after the operation. Since gamma-globulin treatment and splenectomy are sometimes ineffective in ITP, it is advisable to wait an operation until the effects of these treatments are clarified.