Zafirlukast (ZAF) is a leukotriene receptor antagonist used in the treatment of chronic asthma. In this study, a simple and sensitive reversed-phase, high-performance liquid chromatographic method was developed for the determination of ZAF in pharmaceutical formulations and human plasma. Piribedil was used as an internal standard. Analysis was carried out on a Nucleosil C-18 100 angstrom (150 mm x 4.6 mm id, 5 mu m) column with acetonitrile-pH 3.0 acetate buffer (70 + 30, v/v) as the mobile phase at a flow rate of 0.8 mL/min. The peak was detected by an ultraviolet detector set at a wavelength of 240 nm. The retention times were about 3.9 min for piribedil and 5.8 min for ZAF. The developed method was applied to the determination of ZAF in its pharmaceutical formulation and spiked human plasma. For quantification of ZAF in spiked plasma, proteins were precipitated with ethanol before chromatographic analysis. The calibration range was linear from 49.69-437.50 ng/mL in spiked plasma. The absolute recovery from spiked plasma was 98.73 +/- 0.42% at a concentration of 254.78 ng/mL of ZAF. No endogenous substances from plasma were found to interfere.