Since 1979, the potent narcotic analgesic fentanyl and its analogs have been synthesized in clandestine laboratories and sold as heroin substitutes. At least 112 overdose deaths have been associated with their use. In this study, toxicology data, autopsy findings, and coroners' investigative reports were reviewed in order to construct a profile of the typical fentanyl overdose victim and to identify any factors that might heighten the risk of death from fentanyl use. The "typical" fentanyl overdose victim was 32.5 +/- 6.7 years of age (range, 19 to 57 years), male (78%, compared with 22% female), and Caucasian (50%, compared with 29% Hispanic, 20% Black, and 0.9% Asian). With the exception of his or her age, the typical fentanyl overdose victim is quite similar to the typical heroin user. Nearly all the deaths (94%) occurred in California, yet within the state they were widely distributed throughout 17 counties and 44 cities. Pulmonary edema and congestion and needle puncture sites were consistent postmortem findings. No preexisting medical conditions were identified as possible risk factors. Although most of the fentanyl victims had a prior history of intravenous drug use, morphine or codeine were not commonly found, which suggests that the victims had little or no opiate tolerance. Ethanol was present in 38% of the cases and is thought to be a significant risk factor. Mean fentanyl concentrations in the body fluids were quite low: 3.0 +/- 3.1 ng/mL (0.3 +/- 0.31 micrograms/dL) in blood and 3.9 +/- 4.3 ng/mL (0.39 +/- 0.43 micrograms/dL) in urine, measured by radioimmunoassay. Although the potency of the analogs and the purity of street samples varies considerably, it is probably the general availability of the drug rather than the potency of a particular analog that determines the incidence of overdose deaths.