Zafirlukast, Cyclopentyl (3-(2-methoxy-4-((o-tolylsulfonyl) carbamoyl) benzyl)-1-methyl-1H-indol-5-yl) carbamate (I). A high-performance liquid chromatographic (HPLC) reversed-phase rapid resolution method has been developed and validated for estimation of zafirlukast in a pharmaceutical active pharmaceutical ingredient, which was chromatographed on reversed-phase Hypersil Gold C18 , 2.1 x 100 mm , 1.9 µ column using mixtures of acetonitrile/water and the eluents were monitored at different wavelengths. The method was validated statistically for its linearity, accuracy, robustness and precision. The method employed same chromatographic condition as above and validated in terms of linearity, accuracy, precision, limit of detection (LOD), limit of quantification (LOQ), and solution stability. Present work also describes the development and validation of RP- RRLC method for the determination of impurities of zafirlukast.