This study was designed to elucidate if there is a potential hazard of developing neuropsychological deterioration after major cardiovascular surgery. Neuropsychological functions were studied in 49 patients before and after cardiac (coronary artery bypass, cardiac valve replacement, etc.) and thoracic aortic surgery (aortic aneurysm). Neuropsychological examinations using 5 batteries (Mini-Mental State Examination, Cross Cultural Cognitive Examination, Miyake's Verbal Memory Test, Benton Visual Retention Test and Wisconsin Card Sorting Test) were performed before surgery and subsequently at 3 weeks and 8 months after. Twenty-five of 49 patients (51%) showed a significant deterioration at least in one of the test batteries. There was no significant relation between age or educational levels and post-surgical impairment of cognitive function. The impairment of cognitive function was found mainly in the "memory function". Follow up examinations were carried out on 31 of the 49 patients 8 months after surgery. Among these patients, 13 patients did not show any changes in cognitive function throughout the study period. Eleven of the 16 patients who developed a decrease in cognitive function for a certain period after surgery (3 weeks), recovered to the same level at the follow-up examination that it was prior to the surgery. Cognitive function in five patients did not improve and remained at the decreased level. The other two patients whose function was normal at the first post-surgical examination, were found to have developed impairment of cognitive function at the follow up. Although the patients had some deficits on their neuropsychological function, they scarcely complained about any problems in their daily life.(ABSTRACT TRUNCATED AT 250 WORDS)