To study the effect of occupational exposure, smoking, and drinking on lymphocyte DNA damage in bus manufacturing workers, 346 employees (106 women and 240 men) from six job categories (welders, mechanics, painters, and assembling, auxiliary and managerial workers) in a bus manufacturing factory in Guangzhou were included. Significant differences of tail moment among the six job categories were found (P=0.003) with adjustment for age and gender. Smoking increased tail moment significantly (3.14 (2.89-3.40) versus 2.79 microm (2.63-2.97), P=0.023). Analysis of covariance showed that occupational exposure (P=0.001) and smoking (P=0.019) had significant effect on tail moment after adjusting for all factors, whereas age and gender had no effect on DNA damage. Stratified analysis showed that painters (P=0.002), auxiliary workers (P=0.011), and mechanics (P=0.044) had larger tail moments than managerial workers after adjusting for age, gender, smoking, and drinking.