BACKGROUND: Tramadol is an alternative to other opioids for postoperative pain management. This prospective, randomized, double-blind study was designed to compare the analgesic efficacy of patient-controlled tramadol with patient-controlled morphine for postoperative pain after tonsillectomy in children. METHOD: Sixty patients were allocated randomly to receive a patient-controlled analgesia (PCA) with either tramadol (T) or morphine (M), in a double-blind randomized study. When surgery was completed and hemostasis achieved, a standardized loading dose (0.1 mg.kg(-1) in group M, or 1 mg.kg(-1) in group T) was given. Thereafter, the children helped themselves to bolus doses (morphine (0.02 mg.kg(-1)) or tramadol (0.2 mg.kg(-1)) with lock-out times of 10 min without time limit via a PCA device. Scores for pain, sedation, nausea, and the bolus and total PCA doses, hemodynamic parameters and side effects were recorded at 5, 15, 30 min and 1, 2, 4, 6 and 24 h during PCA administration. RESULTS: Pain scores decreased significantly with time in both groups (P < 0.05), but were lower in group M than in group T at 1, 2 and 4 h (P < 0.05). Sedation scores increased with time in both groups (P < 0.05). However there were no significant differences in sedation scores between two groups at any study period, but nausea scores were higher in M group at 4, 6 and 24 h (P < 0.05). CONCLUSION: Intravenous patient-controlled tramadol is an alternative to patient-controlled morphine for postoperative pain relief in children after tonsillectomy. Morphine gave better postoperative pain relief, but was associated with a higher incidence of nausea than tramadol.