A sensitive and selective liquid chromatographic method coupled with tandem mass spectrometry (LC-MS/MS) was developed and validated for the quantification of methanesulfonamide (MSA) in human urine. MSA is a potential in vivo metabolite of reparixin, a specific inhibitor of the CXCL8 biological activity. In this study, a simple derivatization procedure with a new reagent, N-(4-methanesulfonyl-benzoyl)-imidazole, was set up to enable MSA and the internal standard (I.S.), ethanesulfonamide (ESA), to be analysed by LC-MS/MS. After derivatization, samples were evaporated and reconstituted in 30% acetonitrile, aq. MSA and I.S. derivatives were separated by reversed phased HPLC (high performance liquid chromatography) on a Luna 5micro C18 column and quantitated by MS/MS using electrospray ionization (ESI) and multiple reaction monitoring (MR M) in the negative ion mode. The most intense [M-H](-) MRM transition of derivatized MSA at m/z 276.2-->197.2 was used for quantitation and the transition at m/z 290.2-->211.2 was used to monitor derivatized ESA. The method was linear over the concentration range from 1 to 100 microg/ml, with a lower limit of quantitation of 1 microg/ml. The intra- and inter-day precisions were less than 5.5% and 10.1%, respectively, and the accuracies were between -4.0% and +11.3%. The method was successfully applied to quantify levels of MSA in human urine after intravenous administration of reparixin to healthy volunteers.