Objective: To evaluate the association between cumulative exposure to 1,3-butadiene (BD) or styrene (STY) and lung cancer among synthetic rubber industry workers. Methods: Internal Cox regression analyses were performed both with and without natural logarithm transformation of exposure variables and both with and without the inclusion of those unexposed to monomers. Results: Among women, analyses using untransformed BD exposure showed no trend. Analyses using natural logarithm-transformed BD exposure indicated a positive trend when the unexposed were included (P < 0.05) and an inverse trend when the unexposed were excluded (P < 0.05). No exposure-response trends were seen for BD among men or for STY among women or men. Conclusions: These results suggest that the association between BD and lung cancer, seen in some analyses of female employees, is not causal. STY did not appear to be associated with lung cancer. © 2009 by American College of Occupational and Environmental Medicine.