Biological monitoring of styrene exposure among workers in the reinforced plastics industry is widely implemented in the region of Emilia Romagna, Italy. More than 18,000 urine samples measurements of the main metabolites of styrene, mandelic (MA) and phenylglyoxylic acid, were retrieved for the period 1978-1990, and 4689 values of MA in postshift urine samples were analyzed for various variables thought to influence styrene exposure. The job performed was found to be the most important predictor of styrene exposure. Hand laminators had the highest exposure (mean MA 682 mg/g creatinine); spray laminators showed lower values (404 mg/g), while levels in semiautomatic process operators (243 mg/g) were only slightly higher than in nonprocess workers (186 mg/g). The use of ventilation resulted in lower exposure, but differences in average values were not particularly wide. Exposure decreased weakly during the study period in all work categories, but the percentage of measurements exceeding the current biological limit value (900 mg/g creatinine, 1300 mg/l corrected for density) is still very high (20% of measurements among hand laminators in 1990). These results indicate that the control measures implemented are only partially effective for the prevention of styrene exposure.