I describe a new fluorometric method for determination of mandelic and phenylglyoxylic acids in urine, the fluorometry being preceded by extraction into ether and thin-layer chromatography. The chromatographically separated acids were quantitated after conversion to stable highly fluorescent derivatives by treatment with concentrated sulfuric acid. This method proves to be more precise, accurate, and reproducible than the existing colorimetric method. The limit of detection is 2 microgram of either acid per milliliter of urine with a CV of less than 15%. The standard curve for either acid is essentially linear from 2 to 100 microgram/mL of urine, with a correlation coefficient (r) of 0.97. No satisfactory correlation was obtained between the concentrations of either acid metabolite as measured in rat urine by fluorometry and colorimetry. The present method is considered suitable for routine biological monitoring of persons exposed to both high and low concentrations of styrene.