Objective: To determine whether internal jugular venous valves influence inflow pressure during retrograde cerebral perfusion. Design: Prospective study. Setting: Community hospital, university setting, single institution. Participants: Ten patients undergoing reconstructive aortic arch surgery with profound hypothermic circulatory arrest. Interventions: During retrograde cerebral perfusion, inflow pressure was continuously measured at 2 separate sites relative to the left internal jugular venous valve (ie, superior vena cava inflow catheter [infravalvular pressure] and rostral left internal jugular vein [supravalvular pressure]). Measurements and Main Results: Infravalvular pressure of 29.8 ± 3.5 mmHg and supravalvular pressure of 22.7 ± 0.8 mmHg were significantly different (mean difference, 7.1 ± 3.6 mmHg; p = 0.041). In 8 patients, the pressure difference was <6 mmHg; whereas in 2 patients, the pressure difference was >20 mmHg. Bland and Altman analysis revealed 95% limits of agreement on mean bias of −12.9 to 27.8 mmHg. Conclusion: Internal jugular venous valves can obstruct retrograde cerebral perfusion inflow, manifest by an inflow pressure difference between the superior vena cava and internal jugular vein. In the presence of competent internal jugular venous valves, measurement of inflow pressure in the superior vena cava may be an inaccurate estimate of actual cerebral perfusion pressure. Internal jugular vein pressure should be monitored to avoid inadvertent cerebral hypoperfusion. Copyright © 2001 by W.B. Saunders Company