Hippuric acid is the most frequently used biomarker in the biological monitoring of occupational exposure to toluene. This product of solvent biotransformation may be also found in the urine of individuals who have not been exposed to the solvent. A smaller fraction of the absorbed toluene is oxidized to aromatic compounds including ortho-cresol, which is not found significantly in the urine of nonexposed individuals. An analytical methodology whereby gas chromatography with flame ionization detection is utilized for determination of o-cresol in urine of workers exposed to toluene is described. The levels obtained were subsequently compared to hippuric acid levels determined in the same urine samples. The analytical method has demonstrated an adequate precision (intra- and interassay coefficient of variation in the range of 2.4-5.4%) and average recovery of 98%. The samples for o-cresol determination were obtained from workers exposed to toluene in three different industrial activities. The concentration range found in exposed groups varied from <0.21 to 2.8 g/ml. The o-cresol values in the urine did not differ significantly among the exposed groups analyzed at the 5% level. The o-cresol and hippuric acid values found in the urine samples showed a significant correlation at the 1% level. These results may represent an additional contribution to studies for a definitive evaluation of the validity of ocresol as a biomarker of exposure to toluene. </abs>