Background: Pazopanib, a new oral angiogenesis inhibitor, has demonstrated clinical activity against multiple solid tumors and was approved for the treatment of patients with advanced renal cell carcinoma. As an exposure-response relationship has been observed for pazopanib, its therapeutic drug monitoring could be a valuable tool in clinical practice. Therefore, the aim of this study was to develop and validate a selective and precise high performance liquid chromatography-ultraviolet method for the measurement of pazopanib in plasma from patients with cancer. Methods: After liquid-liquid extraction with diethyl ether, pazopanib and gefitinib (internal standard) were separated using isocratic elution on an Ultrabase C18 column using a mobile phase consisting of a mixture in vol/vol proportion of 47:53 of ammonium acetate (pH, 7; 0.02 mol/L) and acetonitrile/methanol (70:30, vol/vol) pumped at a constant flow rate of 1 mL/min. Quantification was performed at 260 nm. Method validation was undertaken as per the guidelines for Bioanalytical Method Validation published by the Food and Drug Administration and European Medicines Agency. Results: Calibration curves were linear over the range 0.5-100 mcg/mL. Interday and intraday coefficients of variations were less than 4.5%. The limit of detection and the lower limit of quantification were 0.2 and 0.5 mcg/mL, respectively. Recovery of pazopanib from plasma was >80%. Conclusions: This is the first high performance liquid chromatography-ultraviolet method for pazopanib quantification that has been validated within a wide range of plasma concentrations and is a suitable method for therapeutic drug monitoring of pazopanib.