Background: Postoperative pain in children is common after tonsillectomy. Rectal diclofenac has been used effectively for postoperative pain management in small children. Aim: To evaluate the prophylactic effect of rectal diclofenac versus intravenous pethidine on postoperative pain management in children undergoing tonsillectomy. Methods: This was a prospective, double-blinded randomized study. A total of 100 children, aged 3-10 years, American Society of Anesthesiologists status I or II, and scheduled for elective tonsillectomy or adenotonsillectomy, were recruited for the study. Patients were randomized to receive either rectal diclofenac 2 mg/kg (group A) or intravenous pethidine 0.5 mg/kg (group B) after induction of general anesthesia. The postoperative pain was assessed using Face, Legs, Activity, Cry, and Consolability scale for the first 6 h. The need for rescue analgesic, rectal paracetamol 40-60 mg/kg, was recorded also after surgery. Moreover, estimation of sedation using Ramsay sedation scale was assessed. Postoperative complications considering mainly vomiting and respiratory depression were recorded. Data were statistically described in terms of mean ± SD, median and range, or frequencies (number of cases) and percentages when appropriate. Results: Pain scores were significantly lower in diclofenac group at different times and needed less rescue analgesic. Postoperative sedation and vomiting were significantly higher in pethidine group. Respiratory depression occurred only in pethidine group. Conclusion: Prophylactic rectal diclofenac is effective in reducing pain after adenotonsillectomy and postoperative analgesic requirement. Moreover, it is generally safe.