The aim of this study is to investigate cancer patients' response and side effects associated with transdermal therapeutic fentanyl (TTS-F), whose pain was hardly controlled by nonweak/weak opioids in Taiwan. From 2005 to 2006, 822 outpatients received TTS-F to collect pain assessment forms and diaries for 4 weeks. Most (78.7%) patients were initially prescribed 25 microg/h TTS-F. Doses were adjusted weekly at clinicians' discretion, according to pain assessment and side effects. Patients receiving 50 microg/h, 75 microg/h, and > 75 microg/h TTS-F had increased from 17.5% to 32.1%, 1.8% to 3.4%, and 1.9% to 2.2%, respectively, by week 2; further small increases were found in weeks 3 and 4. Pain palliation improved from 60.6% during week 1 to 78.6% at week 4. The common adverse effects were nausea/vomiting. Patient's compliance was >90%. This study found that the TTS-F is effective and well tolerated.