Postoperative delirium and abnormal behavior were studied in 160 patients aged 60 and over with relation to their QOL assessed before surgical operation. QOL was assessed physically, psychologically, mentally and socially. Physical status was assessed with ability in daily life, seeing, hearing and severity of disease. Psychological condition (depression) was assessed by using GDS (Geriatric Depression Scale of Sheikh), Mentality (dementia) was assessed by using CDR (Clinical Dementia Rating) and HDS-R (Revised version of Hasegawa's dementia Scale). Sociality was assessed by social life and familial environment. Postoperatively 37.1% of males and 28.9% of females developed delirium and abnormal behaviour. Abnormal behaviour of demented patients was not defined as due to delirium or as dementia itself, so it was included in the classification "delirium and abnormal behaviour" because of the same aspect in terms of practical nursing care. The following factors were found to be statistically related to the occurrence of postoperative delirium and abnormal behaviour: disability in daily life, dementia, disturbance of hearing. Scores of HDS-R was closely related with the possibility of postoperative delirium and abnormal behaviour.