During a fire-year period from March 1979 to February 1984, 559 patients underwent surgical treatment for rheumatic mitral valve disease at Seoul National University Hospital. Left atrial thrombus was found at surgery in 74 of these patients, an incidence of 13.2%. This report outlines the surgical experience with these seventy-four patients. They were 32 men and 42 women, and their age ranged between 23 and 57 with mean of 42 years old. All except one had the findings of atrial fibrillation on the preoperative electrocardiogram. Ten [13.5%] had experienced embolic events before surgery. A history of one or more episodes of cardiac failure requiring digitalis and diuretics was noted in all patients Nineteen were associated with the aortic and/or tricuspid valve disease and closed mitral commissurotomy had been performed previously in four patients. The average cardiac index was 2.68L/min/M2 [1.59 to 3.47 L/min/M2] The mean systolic pulmonary artery pressure was 57mmHg [26 to 120 mmHg]and pulmonary capillary wedge pressure 21.5 mmHg [12 to 40 mmHg]. There were no significant hemodynamic findings to suggest the presence of the left atrial thrombus. Operative mortality was acceptable with two early deaths [2.7%]. The follow-up of the hospital survivors has shown excellent functional status.