Background and Purpose. Jugular venous valve insufficiency may play a role in different neurological diseases. This study describes the methodology to detect internal jugular valve insuf ficiency and establishes functional and morphological criteria to discriminate retrograde flow during valve closure from retro grade insufficiency flow. Methods. Valve closure was assessed in 100 valves (50 healthy volunteers). The valves were visualized in B-mode. During a pressure-controlled Valsalva maneuver, valve closure was monitored by color duplex. The duration and the peak velocity of retrograde flow were determined. Results. Backward flow during valve closure in competent valves was visually clearly discernible from a retrograde flow jet through insufficient valves. Insufficiency was found in 29% of valves. The duration of backward flow in competent valves was between 0.22 and 0.78 seconds (mean = 0.46 ± 0.14 seconds on Dopp ler measurements) and in insufficient valves between 1.23 and 6.15 seconds (mean = 2.66 ± 1.28; P < .0001). Peak velocity of retrograde flow in competent valves was between 12 and 65 cm/s (mean = 26.2 ± 11.1 cm/s) and between 25 and 160 cm/s (mean = 89.5 ± 39.3 cm/s; P < .0001) in insufficient valves. On B-mode imaging, the “typical” aspect of an immobile, frozen valve was seen only in 5 cases of insufficient valves; 21 insuffi cient valves did not display this aspect. Conclusion. The dura tion of retrograde flow clearly discriminates competent and incompetent valves. On the basis of our results, we provide cut off values that help differentiate between physiological and insufficiency reflux. The differences in peak velocity and morphology criteria are helpful but not reliable to predict insufficiency of the valve.