A simple and highly sensitive high-performance liquid chromatographic method for the determination of cholesterol and cholestanol in human serum is described. After extraction of serum with n-hexane, these compounds and 1-eicosanol (internal standard) are converted into the corresponding fluorescent carbamic esters by treatment with 3,4-dihydro-6,7-dimethoxy-4-methyl-3-oxo-quinoxaline-2-carbonyl azide in benzene. The derivatives are separated within 32 min on a reversed-phase column (YMC Pack C8) with acetonitrile-methanol-water (81:9:10, v/v/v) as eluent and detected fluorimetrically. The detection limits are 2 pg (5 fmol) and 3 pg (7 fmol) for cholesterol and cholestanol, respectively, at a signal-to-noise ratio of 2 in a 20-microliters injection volume. This sensitivity permits precise determination of cholesterol and cholestanol in 1-5 microliters of normal human serum.