Cognitive decline and delirium are common complications after heart bypass surgery. Based on the reported role of the APOE-epsilon 4 allele in neurodegenerative diseases, we studied its association with these complications. A neuropsychological test battery consisting of the Mini Mental State Examination, the Wechsler's Memory Scale Revised, the Brief Psychiatric Rating Scale, and the Delirium Rating Scale was applied to 137 APOE-genotyped patients on admission and 1 month after bypass surgery. We correlated the APOE (apolipoprotein E) polymorphism with the postoperative test outcome by taking into account all factors known to influence cognitive capacity after heart surgery. There was a significant decline in all test results 1 month after surgery and a high frequency of postoperative delirium. Neither this decline nor the frequency of delirium was associated with the APOE-epsilon 4 allele. This study confirms the high incidence of cognitive decline and delirium after coronary surgery, but it does not support the role of the APOE-epsilon 4 allele in the occurrence of these complications.