BACKGROUND: This study was designed to compare intramuscular diclofenac with intravenous fentanyl in the prevention of pain after tonsillectomy and adenoidectomy for day-surgery. METHODS: We studied 60 patients, aged 3-13 years, undergoing tonsillectomy with or without adenoidectomy. Patients were randomly assigned to receive placebo injecton (Group 1), intramuscular diclofenac 1 mg/kg (Group 2) or intravenous fentanyl 1 microgram/kg (Group 3) after induction of anesthesia. Pain was evaluated by using an observer pain score for the first 30 min, 60 min and 4 hrs postoperatively. RESULTS: It is statistically significant that group 2 and group 3 had lesser pain than group 1 at 30 min and 60 min. But there is no difference among any groups at 4 hr postoperatively. CONCLUSIONS: We conclude that diclofenac may have advantages compared to fentanyl in safety and convenience for the treatment of pain after tonsillectomy in children.