This paper describes a sensitive biological monitoring method for assessing exposure to styrene. Two major metabolites of styrene, mandelic acid (MA) and phenylglyoxylic acid (PGA), were measured in urine using reversed-phase high-performance liquid chromatography with a variable wavelength UV detector. The urine sample (200 microL) was saturated with 60 mg of sodium chloride and spiked with 20 microL of internal standard (O-methyl hippuric acid). Hydrochloric acid (6N HCl) was added for acidification followed by extraction with ethyl acetate. The extract (0.5 mL) was dried and reconstituted with the mobile phase. The mobile phase used was water-methanol (90:10) with 0.5% acetic acid. The filtrate (5 microL) was injected into the HPLC with a C18 column. The detection limits for MA and PGA were estimated to be 5 mg/L and 0.5 mg/L, respectively. The average recovery was 96% for MA and 84% for PGA. The between-days coefficients of variation for both metabolites were generally less than 11%. The average within-day variations were usually less than 5%. The method was verified with urine samples collected from workers exposed to styrene. Excellent correlations were observed between environmental styrene exposure and urinary MA (r = 0.92) and PGA (r = 0.85), determined by using the present method. The procedure is sensitive and reproducible, and can be applied to occupational health measurement of styrene exposure.