A highly sensitive and specific liquid chromatography coupled with tandem mass spectrometric (LC–MS/MS) method has been developed and validated for the simultaneous determination of lansoprazole and its metabolites 5′-hydroxy lansoprazole and lansoprazole sulphone. The detection was operated with multiple reaction-monitoring (MRM) using the electrospray ionization technique. The assay procedure involved precipitation of plasma samples with acetonitrile after indapamide was added as internal standard (IS). The chromatographic separation was achieved with a mixture of methanol–0.2% ammonium acetate and 0.1% methanoic acid in water (75:25, v/v) as mobile phase on an Inertsil ODS-3 column. The method was proved to be accurate and precise with linearity ranges of 10–4000 ng/ml, 5.0–400 ng/ml, and 1.0–400 ng/ml for lansoprazole, 5′-hydroxy lansoprazole and lansoprazole sulphone, respectively, with the correlation coefficients (r) better than 0.999. The lower limits of quantification (LLOQ) were 2.0 ng/ml, 2.0 ng/ml, and 0.5 ng/ml for lansoprazole, 5′-hydroxy lansoprazole and lansoprazole sulphone, respectively. The intra- and inter-day precision and accuracy values were found to be within the assay variability limits (R.S.D.% within ±15) in accordance with FDA guidelines. The validated LC–MS/MS method has been successfully applied for the determination of lansoprazole and its metabolites in human plasma.