A cross-sectional study was carried out on laminators producing glass-fibre reinforced plastics, to evaluate the role of genetic polymorphism of xenobiotic metabolising enzymes on the genotoxicity of styrene. Clastogenic effects, evaluated by the micronucleus test, are related with end-of-shift urinary concentration of 4-vinylphenol and seem to be modulated by NQO1 polymorphism; aneuploidogenic effects, evaluated by the identification of centromers in micronuclei using the fluorescence in situ hybridisation technique with a pancentromeric probe, are related with before-shift urinary levels of mandelic and phenylglyoxylic acids and seem to be modulated by the GSTM1 polymorphism.