In order to characterize fentanyl-related deaths in the province of Ontario, Canada, a retrospective study of all cases in which fentanyl was quantitated in blood was conducted for the time period between 2002 and 2004. A total of 112 fentanyl-related deaths were identified. Decedents ranged in age from 4 to 93 years and comprised 63 men and 49 women. A variety of routes of administration of the drug were identified: transdermal application of Duragesic patches, intravenous injection of patch contents or fentanyl citrate solution, oral/transmucosal administration, and volatilization and inhalation of Duragesic systems. Blood fentanyl concentrations were determined for all modes of drug administration and are provided. There were 54 cases in which death was attributed solely to fentanyl intoxication; the mean blood concentration was 25 microg/L (range: 3.0-383 microg/L). This concentration range overlapped with blood fentanyl concentrations measured among cases where the presence of the drug was considered incidental. For example, a mean blood concentration of 12 microg/L was observed among 12 cases of natural death (range: 2.7-33 microg/L). Detailed case reports of six individuals are also included and provide additional insight into the use of this drug for both therapeutic and illicit means.