OBJECTIVE To investigate the effects of different factors associated with open heart surgery under cardiopulmonary bypass (CPB) on neurocognitive function. METHODS The cognitive capacity of 50 patients undergoing open heart surgery under CPB were studied on the day before operation and the 14th day postoperatively with Basic Cognitive Capacity Test battery, including 7 subtest: Digit Discrimination, Mental Arithmetic, Chinese Character Rotation, Digit Working Memory, Dual-word recognition, Tri-digit Recognition, and Meaningless Figure Recognition. The correlation between the postoperative cognitive decline and the factors associated with CPB was analyzed. RESULTS The total score of the Basic Cognitive Capacity Test 14 days postoperatively in 31 out of the 50 patients (62%) was 50 +/- 13, significantly lower than the preoperative score (54 +/- 10, P < 0.01), the decline being especially marked in Digit Discrimination, Digit Working Memory, and Dual-word Recognition (all P < 0.01). The postoperative decrease in cognitive function was correlated with age (P < 0.05), CPB time (P < 0.01), rewarming rate (P < 0.01), and the highest PaCO(2) (P < 0.05). CONCLUSIONS The incidence of cognitive dysfunction is higher in patients undergoing open heart surgery with CPB during the early postoperative period. Shortening CPB time, slowing rewarming, and appropriately increasing the level of PaCO2 will help to degrade the possibility of cognitive dysfunction after open heart surgery.