Abstract: Fatalities associated with fentanyl hydrochloride are increasingly seen in Massachusetts. Between September 2005 and November 2006, 5009 medicolegal investigations associated 107 deaths with licit or illicit fentanyl use, along with a co-detection of an opiate/opioid or cocaine/benzoylecognine, or both. Deaths associated with illicit fentanyl use occur in younger people (39.4 vs. 61.5 years) with higher fentanyl (17.1 ng/mL vs. 4.4 ng/mL) and lower morphine (76.9 ng/mL vs. 284.2 ng/mL) postmortem blood concentrations, and more frequent cocaine co-intoxication (65% vs. 3%), than deaths associated with licit fentanyl use. A wide range of postmortem blood concentrations of fentanyl was detected (trace–280 ng/mL), with a minimum concentration of 7 ng/mL of fentanyl strongly associated with illicit use of fentanyl in poly-drug cases. The most commonly detected opiates/opioids in illicit fentanyl users were: morphine (29%), oxycodone (14.5%), and methadone (14.5%). Ethanol, cannabinoids, diazepam, citalopram, and diphenhydramine were each detected in greater than 10% of the illicit fentanyl cases. Most fentanyl abusers died at their own home and their deaths were most often classified as accidental. Mapping of primary residences of decedents revealed conspicuous clustering of the illicit fentanyl use cases, as opposed to the random pattern in licit use cases. Fentanyl misuse is a public health problem in Massachusetts.