The cause of postoperative delirium is unknown. In 7 patients with postcardiotomy delirium (6 men, 1 woman; mean age 67 years), we observed a plasma concentration of tryptophan and a plasma tryptophan ratio significantly lower than in 8 non-delirious postoperative control patients. We suggest that the mental symptoms of postcardiotomy delirium are the consequence of a reduced cerebral tryptophan availability due to a catabolic state.