An exceptional epidemic of poisonings due to the highly potent opioid designer drug 3-methylfentanyl (TMF) was revealed among Estonian drug users in 2005-2006 by post-mortem forensic toxicology. Quantitative analysis of cis-TMF, trans-TMF, and fentanyl was performed by liquid chromatography-tandem mass spectrometry. Comprehensive toxicological analysis was performed using a multi-technique approach. The number of TMF-related fatal accidental poisonings identified was 46 and 71 for 2005 and 2006, respectively. The proportion of male victims was 91.5% and the mean age of all victims was 26 years at death. TMF was used predominantly by intravenous injection. There was no significant difference in the blood concentrations of cis-TMF and trans-TMF between pure TMF poisonings and mixed TMF poisonings. The mean combined concentration of TMF stereoisomers among pure TMF cases (1.9 microg/l) was more than ten times lower than the mean fentanyl concentration in fentanyl-related fatalities. Concomitant use of other drugs involved alcohol, amphetamines, benzodiazepines, and cannabis, but very rarely other opioids.