Background and aim of the study: Tricuspid valve exploration during surgery plays an important role in the decision-making of concomitant tricuspid annuloplasty at the time of left-sided valve surgery. However, at present a good standard to define tricuspid annular dilatation is not available. The study aim was to introduce an alternative method based on armular circumference to judge the extent of annular dilatation, and investigate its predictive ability for the postoperative progression of tricuspid regurgitation (TR). Methods: A total of 127 patients with non-significant TR who underwent isolated left-sided valve surgery at the authors' institution between October 2010 and October 2011 were enrolled prospectively in the study. Intraoperative measurements of the tricuspid annular circumference (TAC) were made for each patient and adjusted to the patient's body surface area to give the TAC index (TACI). The primary end-point was defined as the progression of TR by more than two grades, or a final TR grade at follow up echocardiography. Results: The mean follow up period was 30.2 months (range: 24-37 months). Three variables were found to be associated with postoperative TR progression, including atrial fibrillation, left atrial diameter, and the intraoperatively measured TACI (p = similar to 0.1 in univariate analysis). However, on multiple regression analysis only the TACI (OR 1.586; 95% CI 1.303-1.929; p < 0.001) was significantly associated with TR progression. Based on the receiver-operator characteristic curve, it was possible to derive an optimal cut-off value (83 mm/m(2)) to predict the postoperative development of TR with higher sensitivity and specificity. Conclusion: Among a patient population with predominantly rheumatic left-sided valve disease, the tricuspid annular circumference, when assessed with special sizers, proved to be an ideal method to judge if the annulus would dilate, or not, during surgery. A deduced TACI threshold of 83 mm/m2 was recommended as an indication for prophylactic tricuspid repair.