A simple and highly sensitive fluorometric high-performance liquid chromatographic method was developed for the determination of busulfan in human serum. After busulfan and 1,6-bis(methanesulfonyloxy)hexane as an internal standard were extracted from serum with ethyl acetate, they were derivatized with 2-naphthalenethiol in an alkaline medium. The derivatives were separated by reversed-phase chromatography on a YMC-Pack C₄ column with a mixture of methanol-0.1 M sodium acetate buffer (pH 7.0) (8:2, v/v) as a mobile phase, and were then detected spectrofluorometrically at 370 nm with excitation at 255 nm. Extraction and derivatization efficiencies were 73.9 - 75.1% and greater than 91.1%, respectively. The detection limit for busulfan added to serum was 2 ng (8 pmol) ml^-1 serum (330 fmol on column) at a signal-to-noise ratio of three with a linear relationship over the 10 ng - 3.0 μg ml^-1 (0.04 - 12 μM) concentration range. The accuracy and precision of this method were within 7.0% and 9.3% even at low concentration (10 ng ml^-1). The within- and between-day variations were lower than 9.3% and 10.9%, respectively.