There is evidence that older people in particular have a higher risk of cognitive dysfunction after surgery under general anaesthesia. We have investigated the severity and character of postoperative cognitive dysfunction after major non-cardiac surgery in patients older than 65 yr. Also, cognitive complaints were studied. Cognitive function was assessed using cognitive tests measuring memory and attention, such as ability to shift between two sequences, ability to ward off distractions, simple cognitive speed and speed of general information processing. These tests were performed before, 1 week (short-term) and 3 months (long-term) after surgery. Cognitive performance of the patients was compared with that of healthy subjects not undergoing surgery who were also subjected to repeated cognitive measurements. After 1 week, patients had a poorer performance on tests measuring simple cognitive speed and speed of general information processing. Three months after surgery, patients and controls showed improved cognitive performance compared with the first measurement. These results suggest that major non-cardiac surgery in older patients causes short-term but not long-term cognitive dysfunction. However, after 6 months, 14 of 48 patients (29%) reported having experienced a decline in cognitive abilities after discharge from hospital. Eight of these 14 patients (17%) were still experiencing these cognitive complaints and reported 'not being the same since the operation'. These findings emphasize that cognitive complaints after major surgery may not reflect actual changes in cognitive performance but may be caused by other factors such as depression or awareness of age-related changes.