Abstract: Objective : To evaluate the efficacy and adverse effects of transdermal fentanyl for patients with moder-ate to severe cancer pain. Methods : A total of 537 patients (aged 18-95 years, with an average age of 61.0±13.20) with moderate to severe cancer pain were enrolled in this multicenter prospective trial all over the na-tion. All patients received transdermal fentanyl. Fentanyl patches were replaced every three days. Thedosage was adjusted according to the patient's condition. All patients received transdermal fentanyl for atleast 8 weeks. Analgesic efficacy of transdermal fentanyl therapy was assessed with visual analogue scale(VAS). Patients themselves assessed the pain intensity. Quality of life was evaluated using the trial draft forthe assessment of quality of life for cancer patients in 1990 in China. A score of 1 represents the worst qualityof life, and a score of 5 indicates the best quality of life. The cumulative score of 6 items totaled 30. We ob-served fentanyl-related adverse events, recorded the corresponding manifestation, occurrence time, remis-sion time and relieving factors. Results : Cancer pain was well controlled 1-3 days after administration of trans-dermal fentanyl. The mean scores of pain intensity decreased significantly due to the treatment (P<0.001).One week after the patch placement, the effective rate was 87.94%. The total effective rate was 96.8%. Thequality of life was remarkably improved after the treatment (P<0.001). Common side effects were constipa-tion, nausea, dizziness, vomiting and sedation. Other relatively rare side effects included bradycardia, lowblood pressure, headache, dysuria, and skin allergy. These symptoms were relieved or eliminated after dosage reduction or drug withdrawal. No serious or fatal adverse reactions were observed. Neither physicalnor psychological dependence occurred. Conclusion : Transdermal fentanyl is effective in relieving moderate tosevere cancer pain and can significantly improve the quality of life, with a low incidence of adverse reactions.Transdermal fentanyl can be recommended as one of the first-line drugs for the treatment of patients withmoderate to severe cancer pain.