A sensitive, specific, accurate, fast and reproducible GC/MS-method for the quantitative determination of 11-keto-boswellic acid in human plasma using 18alpha-glycyrrhetinic acid as the internal standard was developed and validated. 11-Keto-boswellic acid and the internal standard were separated from acidified plasma by liquid/liquid extraction. The extracted samples were methylated and analyzed by GC/MS in the negative ion chemical ionization mode (NICI) and selected ion monitoring (SIM). The total run time was 8 min between injections. The assay described in this paper demonstrates a validated lower limit of quantification of 0.0100 microg/ml using 1 ml of plasma. The calibration curves are linear in the measured range between 10.0 and 2000 ng/ml plasma. The overall precision (expressed as CV) and accuracy (expressed as bias) for all concentrations of quality controls and standards is better than 15%. No indications were found for possible instabilities of 11-keto-boswellic acid in plasma, in whole blood, in the extraction solvent or after repeated thawing/freezing cycles. The recovery of the extraction method is calculated as 84%. The assay was applied successfully to determine the plasma level of 11-keto-boswellic acid in a clinical pilot study.