OBJECTIVE The purpose of this study was to examine changes in severity of secondary tricuspid regurgitation (TR) accompanying mitral valve disease, and to identify factors predicting failure of improvement in TR after mitral valve surgery. METHODS We studied 99 patients who had TR of grade ≥2+ preoperatively, 47 with tricuspid annuloplasty (TAP Group) performed concurrently, and 52 with mitral surgery alone (nTAP Group). Predictors of failure of improvement in TR in the nTAP Group were analyzed. RESULTS The mean follow-up period was 4.6 ± 2.7 years. There was a significant difference between the TAP and nTAP Groups in TR improvement (93.6% vs. 67.3% respectively, p <0.001) and in change of TR grade (-2.1 ± 1.0 vs. -0.9 ± 1.0 respectively, p <0.001). Univariate analysis in the nTAP Group identified rheumatic etiology, atrial fibrillation, mitral stenosis (MS), and large left atrium prior to surgery as risk factors for failure of improvement. In multivariate analysis atrial fibrillation was identified as a predictor of failure of improvement (p = 0.004). CONCLUSIONS Our results suggest that TAP should be performed concurrently with mitral valve surgery in patients with secondary TR of grade ≥2+, especially those having atrial fibrillation, even if TR is not severe.