In order to evaluate the genotoxic hazard among workers potentially exposed to low level petrochemical substances, the analyses of micronuclei (MN) and sister chromatid exchanges (SCEs) in lymphocytes were performed in 46 male workers (as exposed group) and 46 nonexposed subjects (as control group). Mean frequencies of MN and SCEs (respectively, 12.9/1000 cells and 6.5/cell) in exposed group were very significantly higher than those (10.2/1000 cells and 5.4/cell) in control group. And there were also significant differences in mean frequencies of MN and SCEs adjusted for age, employment duration, smoking, and drinking between two groups. Median frequencies of MN and SCEs in exposed group were very significantly higher than those in control group. Frequencies of SCEs were higher in smokers than in non-smoker. Frequencies of MN in smokers, however, were similiar to those of non-smoker. Interaction between exposure and smoking on MN and SCEs induction was not observed. The results suggest that there is genotoxic hazard in high risk group like workers handling carcinogens in petrochemical plants and the analyses of MN and SCEs are useful biomarkers for the exposure to hazard substances even at the level below the exposure limit.