The purpose of this prospective multicentre survey was to evaluate the occurrence and the type of changes in children's behaviour during the first 4 weeks following the day of surgery, and to assess the significance of some patient-related factors on the incidence. Pre- and postoperative questionnaires were completed by the parents of 551 children aged 4 months to 13.4 years in five hospitals incorporating nine operative units in Northern Finland. The overall incidence of problematical behavioural changes was 47% and that of beneficial changes 17%. Problematical changes were most common in the 1.0 to 2.9 year olds and the incidence decreased significantly from 46% on the day of the operation to 9% 4 weeks later (p < 0.0001). Predictors by multiple logistic regression analysis were age, mild pain at home following surgery, severe pain and a previous bad experience of health care which had adversely affected the attitude of the child towards doctors or nurses. Hospital influenced playing was a significant factor 3 and 4 weeks after the operation. By the 4th week, beneficial and problematical changes were equally common (9%). Gender, previous operations and experience of repeated paracenteses (for treatment of middle ear infection) did not have a significant effect on the incidence. Pain on the day of the operation predicted the occurrence of behavioural problems up to the 4th week, 2-4 weeks longer than the duration of pain itself. The results emphasise the importance of effective prevention of postoperative pain as well as the importance of avoiding unpleasant experiences in all contacts children have with health care. Playing could perhaps be used to help children cope with a short hospital experience.