Background: Development of late tricuspid regurgitation (TR) after mitral valve surgery is problematic in clinical practice. We investigated the natural history of TR in patients who underwent mitral surgery. Methods: Among 353 patients who underwent mitral valve (짹 aortic valve) surgery between Jan. 1993 and Dec. 1995, patients without follow-up echocardiography (n=141) and patients undergoing tricuspid valve replacement (n=9) were excluded. Therefore, 203 patients (75 male, 128 female; mean age 46 짹 13 yr, range 16 to 81) with long-term follow-up echocardiogram available were included for the analysis. Mean duration of follow-up was 9짹1 years. Results: Among 203 patients, 178 patients (83%) had mitral valve replacement and the others had repair surgery. 56 patients (28%) had combined aortic valve surgery. At the time of mitral valve surgery, 21 patients (10%) had significant (more than mild) TR. At the follow-up exam, 88 patients (43%) showed significant TR; 27 mild to moderate, 24 moderate, 11 moderate to severe, 26 severe TR). Patients with the development of significant TR were older (50 yr vs 43 yr, p<0.05), had higher incidence of atrial fibrillation (81% vs 61%, p<0.05), and showed higher pulmonary artery pressure (PAP) (50 mmHg vs 44 mmHg) compared with patients without significant TR. However, left ventricular ejection fraction and the type of mitral surgery was not associated with TR progression. In logistic regression analysis, preoperative atrial fibrillation was major risk factor in progression of TR (RR 2.9, 95% C.I. 1.3-6.9, p<0.05). Conclusions: Our results showed that a substantial number of patients undergoing mitral valve surgery developed significant TR during follow-up, which was associated with advanced age, preoperative atrial fibrillation and pulmonary hypertension.