The misuse of GHB as a knockout agent continues to increase. A full evaporation technique with headspace gas chromatography and flame ionization detection (FET-HS-GC-FID) method has been developed for the rapid determination of GHB (gamma-hydroxybutyric acid; “liquid ecstasy”) in serum samples. GHB occurs as an endogenous substance in low concentrations in mammals, is used as a recreational drug and misused as an agent in drug-facilitated sexual assaults or robbery as “knock out drops” in higher concentrations. Rapid determination of substances is particularly important in emergency analysis, especially in the case of unconscious patients. The FET-HS-GC-FID method developed in this paper operates with a sample volume of only 8 µL serum and provides slightly better validation data than the comparison method, which works with 200 µL serum. In order to be able to detect the low-volatile GHB, this is in situ converted into the high volatile lactone under acidic conditions. A thermostatting time of 20 min at 130 °C is sufficient for the lactonization. The FET-HS-GC-FID method enables the detection of GHB to GBL (gamma-butyrolactone) with a limit of detection (LOD) of 1.25 mg L⁻¹, and a limit of quantification (LOQ) of 4.26 mg L⁻¹ in serum samples. The method shows a good precision intraday (7.2 and 6.4 %) and interday (7.8 and 5.6 %) for both the low (lQC) and the high quality control (hQC) sample and a calibration curve determination coefficient (R²) of 0.999. The comparison of 100 spiked serum samples with GHB show an excellent agreement between the conventional and the new developed FET-HS-GC-FID method with a R² of 0.984 and the results of the proficiency tests showed a very good agreement with the target values and were in all cases within the assessment limits.