Hydroxyurea is an antitumor drug widely used in the treatment of sickle cell disease. The drug has been analyzed in biological fluids by a number of high-performance liquid chromatography (HPLC) methods. This paper describes a fast and highly reliable capillary gas chromatography–mass spectrometry (GC–MS) procedure that was developed for the detection and quantitation of hydroxyurea in plasma. The compound and its labeled internal standard were liquid extracted from plasma and derivatized with BSTFA before analysis. The detection limit of the assay was 0.078μg/ml and the limit of quantitation was 0.313μg/ml with linearity up to 500μg/ml. Intra-day variation, as coefficient of variation (C.V., %) over the selected concentration range, was 0.3–8.7% and inter-day variation was 0.4–9.6%.