A 59-year-old male underwent mitral valve annuloplasty for mitral valve prolapse. The postoperative course was uneventful. However, three months after operation, he developed general malaise and dyspnea, and was readmitted to hospital. Echocardiography revealed mitral valve regurgitation(IV°), and chest x-ray showed an infiltrative shadow in the right lung. A diagnosis of interstitial pneumonia was confirmed based on an increase in serum KL-6 to 1,400 U/ml, and lung biopsy. Respiratory functions improved after about one month of mechanical ventilation and steroid pulse therapy, but 3 l/min of oxygen by nasal cannulae was still required. Chest x-ray revealed that the pneumonia had stabilized, although the serum KL-6 value remained high. Then, mitral valve replacement was performed. Extubation was done two days after surgery, and his respiration was stable thereafter. The serum KL-6 level decreased from 1,170 U/ml on the day of surgery to 541 U/ml 10 days after surgery.