PURPOSE: This study was carried out to evaluate the role of the septal leaflet of the tricuspid valve. The hypothesis was that the neonatal tricuspid regurgitation (TR) was related to a transient elevation of right ventricular pressure in the neonatal period, that deviated ventricular septum and pull the anchored septal leaflet of the tricuspid valve. The pulled septal leaflet created the incomplete coaptation between the tricuspid valves and the regurgitation. METHODS: We used cross-sectional echocardiography, Doppler and color Doppler echocardiography to measure the normalized ventricular curvature at end-systole, the angle of the septal leaflet of the tricuspid valve, the type of coaptation of the tricuspid valve, the direction of regurgitation and the peak flow rate of regurgitation in 114 normal newborn babies (87 with TR, 27 without TR). RESULTS: The normalized septal curvature ratio (p=.0113), the angle of the septal leaflet (p=.0023) and the corrected diameter of the tricuspid valve annulus (p=.0008) were related statistically to the presence of TR (not in the angle of the anterior septum). But in the TR group, right ventricular pressure (calculated through peak velocity of TR) had no correlation with the above variables except the grade of TR (p=.002). CONCLUSIONS: One of the pathophysiological aspects of the neonatal TR is mainly related to the septal leaflet of the tricuspid valve, but the exact pathophysiology of neonatal TR may also be combined with other factors.