In this study a group of 52 workers employed in a plant manufacturing fiberglass-reinforced plastic (FRP) pipes and cisterns, and therefore daily exposed to styrene, were monitored. As a control group 24 non-exposed workers from another factory producing and repairing pallets volunteered to participate. The airborne styrene during the monitoring ranged from 2.2 to 110.1 mg/m3. As a metabolic marker for styrene exposure mandelic acid was measured in the urine and ranged from 11 to 649 mg/g creatinine. From 43 exposed and 15 control workers sister-chromatid exchanges (SCE) and high frequency cell (HFC) data and from 49 exposed and 23 control workers micronucleus (MN) data from peripheral lymphocytes are reported. Although the two groups of workers could clearly be distinguished on the basis of the airborne styrene concentrations and urinary mandelic acid concentrations no differences in any of the cytogenetic markers were found. Correlations between the cytogenetic data and the level of airborne styrene concentrations or urinary mandelic acid levels could also not be demonstrated. Otherwise, smoking increased the SCE frequency. Grouping the workers according to smoking habits showed a statistically significant difference in SCE. Moreover, levels of urinary thiocyanate (SCN), which can be used as a metabolic marker for smoking, showed a significant positive correlation with the number of SCE. This indicates that SCE is a sensitive biomarker and might still be useful in biomonitoring. However, only chronic exposures over a long period would probably be detectable. In this study, where exposure was rather low and the number of working years was small (mean of 2.9 years), cytogenetic effects are probably too low or rare to be detectable with any assay.