The purpose of this study is to investigate the utility of fentanyl matrix for the treatment of pain due to osteoarthritis (OA) of knee or hip, which was not adequately controlled by non-opioid analgesic (e.g. NSAIDs or COX-II inhibitors) or weak opioids [tramadol HClacetaminophen, tramadol or codeine (or combinations with acetaminophen/ibuprofen)].

This study is a prospective, open-label, non-interventional study. The objective of this study is to investigate the utility of fentanyl matrix after 8 week treatment of fentanyl matrix for the treatment of pain due to osteoarthritis (OA) of knee or hip, which was not adequately controlled by non-opioid analgesic (e.g. NSAIDs or COX-II selective inhibitors) or weak opioids [tramadol HCl with acetaminophen, tramadol or codeine (or combination with acetaminophen/ibuprofen)] in clinical practice according to the investigator's discretion. This study will investigate the usefulness of fentanyl matrix based on improvement in pain and function after Fentanyl matrix administration in patients who complained of pain caused by osteoarthritis and indicated for Fentanyl matrix administration in clinical practice at the investigator's discretion. This study will also measure the patients' functional improvement through Korean Version of WOMAC. Observational study - No investigational drug administered

Inclusion Criteria:

• Patients who complain of chronic pain caused by osteoarthritis of the knee or hip persisting for 3 months or longer
• Patients who can fully observe the overall clinical study requirements including K-WOMAC completion at the investigator's discretion
• Patients who complain of severe pain (NRS score = 7) because pain was not treated enough with the previous analgesic
• Patients who have never been administered strong opioid analgesics over the last one month

Exclusion Criteria:

• Patients with a history of the drug or alcohol abuse in the past or now
• Childbearing women who are pregnant or likely to be pregnant during the study period and male subjects who are neither infertile nor willing to refrain from sexual relations but whose partner does not conduct an effective contraception (implant, injections, oral contraceptives, intrauterine device, etc.)
• Patients who are unable to use a transdermal system due to skin disease
• Patients with serious mental disorder
• Patients with history of hypersensitivity to opioid analgesics
• Patients with history of CO2 retention
• Patients who are not eligible for the study participation based on warnings, precautions and contra medications in the package insert of the study drug at the investigator's discretion

The study population is the patients who visit a center during the study period with the complaint of chronic pain caused by osteoarthritis of the knee or hip, and are deemed to fail to control pain in a proper way and need fentanyl matrix administration according to the investigator's discretion