As part of a major prospective study of neurological and psychological complications of coronary artery bypass graft surgery, involving 312 patients, detailed psychometric testing was carried out before and after operation on 298 patients using a battery of 10 standard tests of intellectual function. This report is concerned with the early neuropsychological dysfunction detectable one week after operation. Two hundred and thirty-five patients (79 per cent of the cohort) showed impairment in some aspect of cognitive function at the seventh day after operation. Only 63 patients (21 per cent) showed no deterioration from levels before operation in any of the 10 test scores. One hundred and twenty-three of the patients whose scores deteriorated had no symptoms while in hospital. Eighty-nine patients complained of cognitive impairment, and 23 patients were considered to be overtly disabled by their intellectual dysfunction, during the period soon after operation. There is therefore a high incidence of early cerebral dysfunction detectable by psychometric testing following coronary artery bypass graft surgery. Often this was not of sufficient severity to cause serious concern to the patients or to interfere with their everyday activities in the hospital environment.