Introduction: It has been known that the posterior leaflet of the tricuspid valve (PTL) has several indentations or clefts in its free edge, giving the tricuspid valve (TV) equal to or more than four leaflets appearance. However, little is known about whether the indentations or clefts can be a location of tricuspid regurgitation (TR) jet origin in severe TR. Hypothesis: This study aimed to investigate the association between quadrileaflet TV and location of TR jet origin in patients with functional severe TR. Methods: We retrospectively studied 85 functional severe TR patients who underwent 3D transesophageal echocardiography which was performed using the EPIQ7 ultrasound system with a X8-2t probe (Philips Medical Systems). The morphology of the TV leaflets was ascertained using 3D volume-rendered images. Clefts extending over 50% of the depth of the leaflets were determined as a landmark between 2 adjacent leaflets. The location of TR jet origin and vena contracta area (VCA) were ascertained using real-time 3D color Doppler images. All images were analyzed by using QLAB 3D quantification software (Philips Medical Systems). Results: In 85 functional severe TR patients, a total of 3 (4%) patients with bileaflet, 60 (71%) patients with trileaflet and 22 (26%) patients with quadrileaflet were identified. All quadrileaflet TV had dichotomous PTLs (PTL 1 and PTL 2) and there was no patient who had equal to or more than five leaflets. In 22 patients with quadrileaflet, 10 (45%) patients showed TR jet origin located between PTL 1 and PTL 2 (Figure A). Also, quadrileaflet TV patients with TR jet origin located between PTL 1 and PTL 2 had significantly larger VCA than those without (185 vs. 62 mm2, P<0.001, Figure B). Conclusions: In patients with functional severe TR, one fourth patients showed quadrileaflet TV morphology. Furthermore, TR jet origin located between PTL 1 and PTL 2 can be seen only in a condition of TR beyond severe.