BACKGROUND: Cognitive dysfunction after open heart surgery under the cardiopulmonary bypass is one of major problems in modern cardiac surgery. We designed this study to evaluate the incidence of postoperative neuropsychological changes after routine cardiac surgery with the cardiopulmonary bypass. METHODS: We assessed the patients with four cognitive function tests on the day before, 4-5 days after and 9-10 days after cardiac surgery. The test batteries which used in this study were Trail-Making Test part A, Digit Substitution test, Digit Span Test, Grooved Pegboard test. RESULTS: The incidence of cognitive dysfunction was 12.8-25% at postoperative 4-5 days, 2.3-11.5% at postoperative 9-10 days. About 80% of patients who showed cognitive dysfunction at postoperative 4-5 days had gained normal cognitive function score at postoperative 9-10 days. CONCLUSIONS: The incidence of cognitive dysfunction after heart surgery is not low, and we must try to find how to reduce these complications.