A selective and precise high-performance liquid chromatography ultraviolet method was developed and validated for the determination of lapatinib in human plasma. After protein precipitation with acetonitrile, lapatinib and sorafenib were separated using isocratic elution (on a C18 Ultrabase column using a mobile phase of acetonitrile/20 mM ammonium acetate in a proportion 53:47 (v/v) pumped at a constant flow rate of 1.2 mL/min). Quantification was performed at 260 nm. Calibration curves were linear over the range 0.2-10 µg/mL. Inter- and intraday coefficients of variation were less than 7%. The limit of detection and the lower limit of quantification were 0.1 and 0.2 µg/mL, respectively. Recoveries of lapatinib from plasma were higher than 86.7% in all cases. The assay was applied to the determination of the drug in the plasma of 2 cancer patients receiving lapatinib, 1000 and 1250 mg orally, and could be useful for therapeutic drug monitoring of lapatinib in routine clinical practice.