Background: The geometric and hemodynamic determinants of functional tricuspid regurgitation (FTR) have yet to be clearly demonstrated. The geometric alteration of tricuspid valve can be accurately determined by 3-dimensional echocardiography. Methods: In 54 patients with various degree of FTR, real-time 3-dimensional echocardiography (SONOS 7500) was performed to obtain real-time zoom 3D image of the tricuspid valve. The angles between tricuspid annulus plane and 3 leaflets (anterior: A慣, posterior: P慣, septal: S慣), septal-lateral and anterior-posterior annulus diameters, and tricuspid tenting volume were measured at a mid-systole frame using Tomtec software. In 39 patients with atrial fibrillation, 3 measurements in 3 different beats were averaged. Pulmonary artery systolic pressure (PASP) was assessed by the maximal velocity of FTR, and the severity of FTR was quantified by the ratio of distal jet area to right atrial area (DJ/RA) using color Doppler 2D echocardiographic study. Results: In patients with mild (DJ/RA < 0.2) and moderate FTR (0.2 ��� DJ/RA < 0.5), P慣 and S慣 were significantly larger than A慣, whereas in patients with severe FTR (DJ/RA ��� 0.5), only S慣 was significantly larger than A慣. All the geometric variables and PASP showed significant correlations with DJ/RA. By multiple stepwise regression analysis, S慣 (p<0.005), septal-lateral annulus diameter (p<0.01) and PASP (p<0.05) were the independent determinants of the FTR severity. In 15 patients with sinus rhythm, septal-lateral annulus diameter was the only determinant, while S慣 was in those with atrial fibrillation. The tricuspid tenting volume showed an excellent correlation (r=0.84, p<0.001) with, and was the only determinant of distal jet area of FTR. Conclusion: The severity of FTR is mainly determined by tethering of the septal leaflet, septal-lateral annular dilatation and the severity of pulmonary hypertension.