The anatomic appearance and competency of internal jugular valves were investigated in both cadavers and surgical patients. In 20 cadavers, the internal jugular vein (IJV) was dissected and the appearance of the valves observed. In two adult patients, the movements of the valves were observed by endoscopic visualization using an intravascular fiberscope, ultrasound techniques, and invasive venography. Transvalvular pressure gradients were measured in 10 adult surgical patients to examine the competence of the jugular venous valve. Nineteen internal jugular valves obtained from the 20 cadavers were situated directly above the termination of the IJV in the inferior bulb and were usually bilateral. The remaining 21 valves could not be examined since they had been already dissected out for another anatomical studies. The valves in 16 of the subjects were bicuspid and semiluminar. Two valves had a single cusp. The opening and closing of the valve was visualized easily with both a fiberscope and real-time ultrasound technique. Eight patients with normal central pressure had competent valves during cough-induced transvalvular pressure gradients of 45.3 ± 10.1 (mean ± SD) mm Hg. Four valves from two cadavers remained competent at 75.4 ± 18.2 mm Hg, but became incompetent at 5.6 ± 4.8 mm Hg after a hole was made with a 14-gauge needle. Our results indicate that the IJV valve is located 0.5–2.0 cm above the union of the subclavian vein and IJV, and plays an important role in preventing retrograde blood flow to the brain. To avoid injury of this valve, we recommend that venipuncture be performed at the level of the cricoid ring or higher in the neck.