Fentanyl is a synthetic opioid that can be delivered through a transdermal therapeutic system (TTS). The aim of this study was to assess the efficacy of fentanyl TTS in treating oral mucositis pain in 75 adult hematopoietic stem cell transplant (HSCT) patients. The analysis was based on 62 patients who developed mucositis. Pain control was assessed by the patients using a visual analogue scale (VAS) from day 0 to day +33 after HSCT. Fentanyl TTS was administered at the patient's request. In all, 20 patients did not require fentanyl (group A). The first 22 patients asking for the patch received fentanyl 25 microg/h (group B) and the subsequent 20 patients received 50 microg/h (group C). There were no significant differences in pain relief between groups B and C. The expected effect of a decrease in mean pain score (mean of the VAS scores of all of the patients in the same group each day) following the application of fentanyl TTS was not noted. We can conclude that fentanyl TTS at the doses used in this study may not adequately relieve oral mucositis pain.