Omeprazole is a benzimidazole compound that acts as a proton-pump inhibitor. Because the metabolism of omeprazole is mainly catalyzed by cytochrome P-450 (CYP) 3A4 and CYP2C19. the genetic polymorphism of CYP2C19 could be of clinical concern in the treatment of acid-related diseases with omeprazole. Therefore, a reliable method for omeprazole phenotyping is desirable in clinical situations. This study has demonstrated the determination of omeprazole and its metabolites in human plasma by liquid chromatography-three-dimensional quadrupole mass spectrometry with a sonic spray ionization interface. The analytical column was YMC-Pack Pro C18(50x2.0 mm I.D.) using acetonitrile-50 mM ammonium acetate (pH 7.25) (1:4) at a flow-rate of 0.2 ml/min. The drift voltage was 30 V. The sampling aperture was heated at 110 degrees C and Shield temperature was 230 degrees C. In the mass spectrum, the molecular ions of omeprazole, hydroxyomeprazole and omeprazole sulfone were clearly observed as base peaks. This method is sufficiently sensitive and accurate for pharmacokinetic studies of omeprazol.