Mandelic acid (MA) is an important metabolite of styrene. In humans, measurement of its concentration in urine provides an important assessment of the overall level of styrene exposure in workers of the reinforced plastic manufacturing industry. The aim of our study was to investigate in these workers the relationship between MA concentration and styrene exposure time and intensity as well as its dependence on work occupation. The concentration of MA in the urine samples of 35 employees was analyzed with HPLC (high performance liquid chromatography). Out of 35 workers, 11 performed laminating, 11 milling and finalizing, 6 laying-up and spraying-up, and 7 worked in background support. Urinal samples were obtained twice a day over the course of three weeks, at the beginning and the end of the work shift. We found a significant increase in MA concentrations during a work shift in all tested days (Wilcoxon test p < 0.05). Employees working in elevated atmospheric concentrations of styrene (93.77-159.88 mg/m3) had significantly higher MA concentrations in urine compared to other groups at both the beginning and the end of the shift (Kruskal Wallis test p < 0.001) (p < 0.001). Only samples from laminating workers exceeded the biological limit of MA concentration (640 mg/L) at the end of the shift. Normalisation of MA concentration to body mass index (BMI, normal range: 21.7 +/- 3.2 kg/m2) refined differences within groups (Kruskal-Wallis analysis p < 0.001). The accumulation of MA at the end of the work shift for measured time period was not significant for the measured time period (Friedman analysis p > 0.11). Our results confirmed that MA is a sensitive metabolic marker of styrene exposure without cumulative effect. However, normalization of MA concentrations to BMI can improve the accuracy of styrene exposure estimates in certain groups of employees.