A study was performed on the detection, separation and quantification of isomers from the new designer drugs named fluoroamphetamines (FAs) in forensic cases in eastern Denmark. The drugs were detected in whole blood extracts by ultraperformance liquid chromatography with time of flight mass spectrometer (UPLC-TOF-MS) and thereafter verified and quantified by UPLC tandem mass spectrometer (MS/MS). The quantitative method involved liquid–liquid extraction of FAs from whole blood, evaporation of organic solvent, and reconstitution with a mobile phase mixture. Identification of the FAs was achieved by the retention time, multiple reaction monitoring (MRM) traces [154 > 109 (quantifier); 154 > 137], and ion ratio of the two transitions. For all FAs, LOQ was 0.002 mg/kg with linear ranges from 0.002 to 1.0 mg/kg whole blood. Since 2008, a total of 15 forensic investigations, mainly driving under the influence of drugs (DUID) cases, involving 4-fluoroamphetamine (4-FA) have been observed with whole blood concentrations ranging from 0.006 to 0.58 mg/kg. One autopsy case involved 4-FA; however, it was determined to be a combined intoxication. In 2010, ortho-fluoroamphetamine (2-FA) was discovered in forensic samples by the same UPLC/MS/MS method and MRM functions because of variation in retention time and ion ratio. Up to now, three eastern Danish DUID cases have involved 2-FA. The whole blood concentrations of 2-FA were 0.028, 0.041 and 0.37 mg/kg, respectively. Thirteen cases with 4-FA and the three cases with 2-FA also contained amphetamine, but no correlation was observed between the amount of FA and amphetamine. So far, 3-FA has not been observed in any cases, and although it co-elutes with 4-FA, 3-FA will be identified by its variation in ion ratio. To our knowledge, this study has confirmed 2-FA in blood from DUID cases for the first time, and provides typical whole blood concentrations of FAs in forensic cases.