Objective: Inadequate pain management has been estimated to occur in most cancer patients. Fentanyl transdermal therapeutic system (TTS) provides continuous controlled systemic delivery of fentanyl for up to 72 hours. In order to assess the efficacy and tolerability of TTS-fentanyl, an open, multicentered, uncontrolled phase IV study was conducted in patients with cancer-related pain. Material and Methods: One hundred and forty eight strong opioid-naive patients started with the lowest TTS-fentanyl dosage available (25 mu g/hr). A dose increase was allowed every third day if needed. Primary efficacy measurements were total dose of TTS-fentanyl, Visual Analoge Scale (VAS) score in the patient dairy and overall evaluation of the pain treatment. Secondary efficacy measurements were safety concerns. Results: Of the 148 patients enrolled, 79 (53.4%) were recorded as opioid tolerant and 69 (46.6%) patients as strong opioid-naive. Pain control, side effects and overall impression improved from visit I to Visit 3 (p< 0.0001). Most patients rated the convenience of the patches as excellent. The most frequent mentioned adverse events were nausea (32.4%) and vomiting (18.9%). Only 37.2% of the patients exhibited adverse events, which were related to the study drug. Conclusion: Long-term treatment with transdermal fentanyl was safe and tolerable to many cancer patients.