N-acetyltrasferase-2 (NAT2) and microsomal epoxide hydrolase (mEH) are polymorphic genes that metabolize different tobacco carcinogens. Smaller studies found inconsistent relationships between NAT2 or mEH polymorphisms and lung cancer risk. To determine whether there is gene-environment interaction between NAT2 polymorphisms, alone or in combination with mEH polymorphisms, and cumulative smoking exposure in the development of lung cancer, we conducted a case control study of 1115 Caucasian lung cancer patients and 1250 spouse and friend controls. The results were analyzed using generalized additive models and logistic regression, adjusting for relevant covariates. There was no overall relationship between NAT2 genotype and lung cancer risk; the adjusted odds ratio (OR) of the rapid versus slow acetylator genotypes was 0.96 [95% confidence interval (CI), 0.79-1.16]. However, gene-environment interaction analyses revealed that the adjusted ORs increased significantly as pack-years increased. For nonsmokers, the fitted OR was 0.66 (95% CI, 0.44-0.99), whereas for heavy smokers (80 pack-years), the OR increased to 1.22 (95% CI, 0.89-1.67). When comparing the extreme genotype combinations of the NAT2 rapid acetylator, higher mEH activity genotype to the NAT2 slow acetylator, and very low mEH activity genotype, the corresponding ORs at 0 and 80 pack-years were 0.30 (95% CI, 0.14-0.62) and 2.19 (95% CI, 1.26-3.81), respectively. Results were similar with ORs derived from stratified models. In conclusion, NAT2 rapid acetylator genotypes are protective against lung cancer in nonsmokers but are risk factors in heavy smokers. The joint effects of NAT2 and mEH polymorphisms are consistent with an independent, additive effect of these two genes, modified by smoking history.