We have developed a simple and rapid method for measuring 4-O-methylpyridoxine (MPN) in the serum of an acute poisoning patient by high-performance liquid chromatograph (HPLC) with a diode-array detector. MPN, a poisonous ingredient of ginkgo seeds was extracted using an Isolute C18 solid-phase extraction column. Equal volumes of the extract were injected into a C18 HPLC column using acetonitrile-10 mM phosphate buffer (pH 3) as the mobile phase. Excellent linearity was obtained over the range of 1-1000 ng at a wavelength of 290 nm, and its detection limit of approximately 0.5 ng was considered to be satisfactory. The mean recovery of MPN in the serum was 92.9 +/- 6.1%. The method was used successfully in a case of acute poisoning.