Background The present study was conducted among Chinese workers employed in glue-and shoe-making factories who had an average daily personal benzene exposure of 31 AE 26 ppm (mean AE SD). The metabolites monitored were S-phenylmercapturic acid (S-PMA), trans, trans-muconic acid (t,t-MA), hydroquinone (HQ), catechol (CAT), 1,2,4trihydroxybenzene (benzene triol, BT), and phenol. Methods S-PMA, t,t-MA, HQ, CAT, and BT were quantiยฎed by HPLC-tandem mass spectrometry. Phenol was measured by GC-MS. Results Levels of benzene metabolites (except BT) measured in urine samples collected from exposed workers at the end of workshift were signiยฎcantly higher than those measured in unexposed subjects (P `0.0001). The large increases in urinary metabolites from before to after work strongly correlated with benzene exposure. Concentrations of these metabolites in urine samples collected from exposed workers before work were also signiยฎcantly higher than those from unexposed subjects. The half-lives of S-PMA, t,t-MA, HQ, CAT, and phenol were estimated from a time course study to be 12.8, 13.7, 12.7, 15.0, and 16.3 h, respectively. Conclusion All metabolites, except BT, are good markers for benzene exposure at the observed levels; however, due to their high background, HQ, CAT, and phenol may not distinguish unexposed subjects from workers exposed to benzene at low ambient levels. S-PMA and t,t-MA are the most sensitive markers for low level benzene exposure. Am.