A liquid chromatography-tandem mass spectrometric method (LC-MS/MS) for quantifying ilaprazole and its major metabolite in human plasma was developed and validated. Sample preparation was based on chloroform extraction. Chromatography was performed on a C18 analytical column and the retention times were 1.2, 1.28 and 1.65 min for ilaprazole, ilaprazole sulfone and omeprazole (internal standard), respectively. The ionization was optimized using ESI (+) and enhanced selectivity achieved by tandem mass spectrometric analysis. The calibration curve ranged from 0.36 to 368.40 ng/mL and from 0.125 to 128.000 ng/mL for ilaprazole and ilaprazole sulfone, respectively. The intra-and inter-day precision (CV) were within 15%, and accuracy of ilaprazole and ilaprazole sulfone were within 80% to 120%. The analytes showed stable over the timescale of the whole procedure. The robustness of the method was demonstrated by good reproducibility of results obtained from the analysis of clinical samples. A liquid chromatography-tandem mass spectrometric method (LC-MS / MS) for quantifying ilaprazole and its major metabolite in human plasma was developed and validated. Sample preparation was based on chloroform extraction. Chromatography was performed on a C18 analytical column and the retention times The ionization was optimized using ESI (+) and enhanced selectivity achieved by tandem mass spectrometric analysis. The calibration curve ranged from 0.36 to 368.40 ng / mL and from 0.125 to 128.000 ng / mL for ilaprazole and ilaprazole sulfone, respectively. The intra-and inter-day precision (CV) were within 15%, and accuracy of ilaprazole and ilaprazole sulfone were within 80% to 120%. The analytes showed stable over the timescale of the whole procedure. The robustness of the method was demonstrated by good reproducibility of results obtained from the analysis of clinical samples.