A liquid chromatography/tandem mass spectrometry (LC-MS/MS) method was developed and validated for the simultaneous determination of pioglitazone (PIO) and its two metabolites: M-III (keto-derivative) and M-IV (hydroxy-derivative) in human plasma. Human plasma samples of 0.2 ml were extracted by a single step liquid-liquid extraction procedure and analyzed using a high performance liquid chromatography (HPLC) electrospray tandem mass spectrometer system. The compounds were eluted isocratically on a C-18 column, ionized using a positive ion atmospheric pressure electrospray ionization source and analyzed using multiple reaction monitoring mode. The ion transitions monitored were m/z 357-->134 for PIO, m/z 371-->148 for M-III, m/z 373-->150 for M-IV and m/z 413-->178 for the internal standard. The chromatographic run time was 2.5 min per injection, with retention times of 1.45, 1.02 and 0.95 min for PIO, M-III and M-IV, respectively. The calibration curves of pioglitazone, M-III and M-IV were well fit over the range of 0.5-2000 ng/ml (r(2)>0.998759) by using a weighted (1/x(2)) quadratic regression. The inter-day precisions of the quality control samples (QCs) were </=10.5% (N=15), coefficient of variation (CV) and the inter-day accuracy (%Nominal) ranged from 84.6 to 103.5% for PIO, 94.4 to 104.0% for M-III, and 96.8 to 101.0% for M-IV. All three analytes demonstrated acceptable short-term, long-term, and freeze/thaw stability. The method is simple, rapid and rugged, and has been applied successfully to sample analysis for clinical studies.