We performed a prospective study of patients with chronic right ventricular (RV) dilation to determine the factors associated with the degree of functional tricuspid regurgitation (FTR). We prospectively enrolled 64 patients with chronic RV dilation and measured right atrial (RA) area, RV area and its fractional area change (RVFAC), tricuspid annular diameter and contraction, tricuspid valve (TV) tethering area, and systolic pulmonary artery pressure. We also measured the RV eccentricity index and the RV sphericity index for simple presentation of RV geometry. Regurgitant orifice area of FTR was obtained for the quantification of FTR. End-systolic RV eccentricity index (r=0.73), end-diastolic RV eccentricity index (r=0.56), RA area (r=0.49), TV tethering area (r=0.37), age (r=0.31), end-systolic (r=0.42) and end-diastolic (r=0.30) tricuspid annular diameters, and left ventricular ejection fraction (r=-0.37) were significantly related to the regurgitant orifice area of FTR in univariate analysis. However, RV area, RVFAC, and systolic pulmonary artery pressure were not. In multivariate analysis, the end-systolic RV eccentricity index (p2.0, 69% and 73% with an end-systolic tethering area>1.0 cm2, and 64% and 59% for an end-systolic tricuspid annulus diameter>3.9 cm, respectively. FTR severity was found to show the best correlation with the end-systolic RV eccentricity index. In conclusion, these findings underscore the importance of eccentric RV dilation for determining FTR severity and should lead to the development of more rational surgical approaches to FTR beyond TV annuloplasty.