Objective: To investigate preoperative clinical conditions and/or intraoperative physiologic variables related to jugular venous oxygen saturation (Sj0 2) during cardiopulmonary bypass (CPB). Design: Prospective study. Setting: General hospital, single institution. Participants: One hundred forty patients (52 women, 88 men) who underwent coronary artery bypass grafting. Measurements and Main Results: The authors measured Sj0 2 at five times during surgery. Multiple stepwise regression analysis showed a significant correlation of SjO 2 with (1) arterial carbon dioxide partial pressure (PaCO 2) before CPB (standard regression coefficient [(SC)] = 0.435), (2) cerebral perfusion pressure (CPP) during initiation of CPB (SC = 0.259), (3) PaCO 2, tympanic temperature (TT), bubble oxygenator, and cerebral small infarctions (CSIs) during hypothermic CPB (SC = 0.507, −0.237, −0.192, and −0.189, respectively), (4) CPP, PaCO 2, CSIs, and bubble oxygenator during rewarming (SC = 0.476, 0.294, −0.220, and −0.189, respectively), and (5) PaCO 2 after CPB (SC = 0.480; p < 0.01). Correlation coefficients between Sj0 2 and CPP during rewarming were 0.40 (0.46 without CSI and 0.37 with CSI; p < 0.01). These results indicate that the relationship between CPP and SjO 2 was significant in patients with CPP less than 40 mmHg during rewarming. Conclusion: During rewarming, when cerebral perfusion and oxygen demand change abruptly, but not during stable hypothermic CPB, CPP was a significant factor related to SjO 2.