The objective of this study is to establish the analgesic efficacy, safety and pharmacokinetic profile of Durogesic® in the treatment of pediatric patients with chronic pain requiring long-term opioid therapy. Patients may be in-patients or treated at home. This study consists of three phases: a pre-trial phase, a treatment phase, and a follow-up phase. During the pre-trial phase, patients receive immediate-release oral morphine or sustained-release morphine (minimum daily dose of morphine is 30 mg/day) for at least 48 hours immediately prior to entry into the study. Patients achieving adequate pain relief in the pre-trial phase will enter the treatment phase and will be switched over to Durogesic®. The dose of Durogesic® will be calculated from the patient's daily morphine requirement, using the conversion regimen. The minimum starting dose is one patch with a fentanyl delivery rate of approximately 25 micrograms/hour. Titration in steps of 25 micrograms/hour is permitted to achieve adequate pain control. The maximum allowable dose is 300 micrograms/hour. The Durogesic® patch is applied to an area of flat skin on the upper body or upper arm, and replaced every 72 hours. The treatment phase of this study will last for 15 days (5 consecutive patch changes). Immediate-release morphine is available as rescue medication to treat breakthrough pain. Upon completion of the treatment phase, the patient may continue to receive Durogesic® therapy in the long-term follow-up phase, as long as the patient, parent or the investigator feels that it is beneficial. The primary measures of effectiveness include the patient's assessment of treatment at Day 15, pain level assessed by the patient twice daily, once in the morning and in the evening (or assessed by the parent if the patient is younger than age 5), the amount of rescue medication that the patient requires, and the pain level at the time rescue medication is given and one hour afterwards. In patients where venous access (either central or peripheral) is available, or if blood is being taken for some other purpose, blood samples will be taken to determine serum fentanyl concentrations. Safety evaluations will include the incidence of adverse events, clinical laboratory tests, assessment of the skin at the site of patch application, and vital signs. Durogesic® will be applied to the upper body or upper arm, and replaced every 72 hours. The minimum starting dose is one patch with a fentanyl delivery rate of approximately 25 micrograms/hour, titrated to achieve adequate pain control, up to a maximum dose of 300 micrograms/hour. The treatment phase is 15 days.