A high-performance liquid chromatographic (HPLC) method is described for the determination of busulfan in human serum using on-line derivatization and column switching. Busulfan was extracted from serum with a mixture of diethyl ether and dichloromethane. After the evaporation of the organic layer, the reconstituted residue was injected into the HPLC system and busulfan was derivatized with sodium diethyldithiocarbamate on the first short column. The back-flushed derivative was then separated on the second column. Finally, after column switching, the heart-cut fraction containing the derivative was further analysed on the third column and monitored with ultraviolet absorbance detection at 278 nm. The lower limit of quantitation in serum was 10 ng/ml.