Fentanyl patches are used by many cancer patients. The liquid content is easy to recover and therefore to abuse. Both of the cases reported here involved male drug addicts, who were found dead within 1 week in the same apartment. In both cases, postmortem femoral blood was screened for amphetamines, cannabinoids, cocaine, and opioids with immunological methods (EMIT II) and with headspace gas chromatography for alcohol and with liquid chromatography mass spectrometry for various drugs, including fentanyl. Confirmatory analysis of fentanyl and morphine was performed by gas chromatography-mass spectrometry. In the first case, the cause of death was determined to be a combination of fentanyl, morphine, and ethanol intoxication, and opioids were considered the most important agents. In the second case, the cause of death was determined to be fentanyl intoxication. The authors advise that physicians should be aware of the potential for misuse of both used and new transdermal patches, their complications, and the risk of fatal outcome. An effective disposal policy should be implemented in places where transdermal fentanyl is used, and patches should be removed from deceased patients and destroyed. Those involved in death investigations should consider the possibility of intravenous use in deaths associated with fentanyl when needle marks are found. The potential for transdermal or oral abuse should also be considered. 13 references