Pulmonary thromboendarterectomy is an innovative surgical technique for treating pulmonary hypertension caused by chronic thromboembolism. The procedure uses cardiopulmonary bypass but necessitates dramatically longer bypass times than coronary artery bypass grafting or valve operations. We prospectively evaluated 22 patients undergoing pulmonary thromboendarterectomy to determine the incidence of delirium and its relationship to certain preoperative and postoperative medical variables as well as to duration of cardiopulmonary bypass, deep hypothermia, and circulatory arrest. Delirium occurred in 77% of patients with a peak incidence around 72 hours postoperatively. No preoperative or postoperative medical variable distinguished delirious from nondelirious patients. Total bypass time was not associated with delirium, but deep hypothermia time and total circulatory arrest time were both strongly associated. Overall prediction of delirium was 81% when total circulatory arrest time was considered. Further analysis suggested that a total circulatory arrest time greater than 55 minutes was both sensitive to (82%) and specific for (80%) delirium. Implications for the mechanism of postcardiotomy delirium and future research directions are discussed.