Background: Pain is a common complaint after adenoidectomy. Behavioral changes after adenoidectomy in children have been reported, and it has been concluded that postoperative pain significantly affects the occurrence of behavioral changes. Behavioral changes, when a proactive pain treatment has been used, have not been systematically studied. Objective: To assess postoperative behavioral changes in children who have undergone day-case adenoidectomy with proactive pain treatment. Design: Prospective, longitudinal, randomized clinical trial. Settings: Ambulatory Care Unit, Department of Otorhinolaryngology, Kuopio University Hospital, Kuopio, Finland. Patients: Three hundred consecutive children, aged 1 to 10 years, who underwent day-case adenoidectomy during 1999 through 2000. Main Outcome Measures: The number of postoperative behavioral changes were evaluated with 3 consecutive questionnaires, at baseline before surgery, 1 week after surgery, and 3 weeks after surgery. Results: A total of 294 questionnaires (98%) were returned after 1 week and 255 questionnaires (85%) after 3 weeks. Most children (91%) had pain after discharge and the mean for pain cessation was 3 days (range, 0-8 days). The mean of ketoprofen doses after discharge was 6 (range, 1-24 doses). Most of the children showed no or only trivial postoperative behavioral changes, and, furthermore, at 3 weeks, more positive than negative changes were reported. The child's age was a significant factor (P<.05) in affecting behavioral changes for all domains. Other significant factors were the worst pain at rest (P=.04) and during swallowing (P=.02) for daytime function disturbances, and fear of separation from parents (P=.03) for sleep disturbances. Conclusion: Day-case adenoidectomy with proactive pain treatment seems to result in a negligible incidence of behavioral troubles in children.