The gene-environment associations between potential carcinogenic agents modified by polymorphisms in the cytochrome P450 1A1 (CYP1A1) gene and lung cancer risk were assessed in a hospital-based case-control study composed of African- and Mexican-Americans. The study involved 171 cases and 295 controls identified from the greater Houston and San Antonio metropolitan areas. Both the exon 7 and MspI polymorphisms were analyzed by RFLP of PCR-amplified DNA, and in addition, the African-American-specific polymorphism was assayed for subjects who reported that they were African-American. Logistic regression analysis was performed to assess the association between each of the CYP1A1 polymorphisms and lung cancer, adjusting for the matching variables (age, sex, ethnicity) and other potential risk factors. Interactions between pack-years smoked, CYP1A1 genotypes, and case status were also evaluated. The variant allele frequencies did not differ by case status, but the distributions of genotypes were strikingly different by ethnicity. In addition, both the exon 7 and MspI polymorphisms, but not the African-American-specific polymorphism, were modified by the amount of cigarette consumption measured in pack-years. An approximate 2-fold increase in lung cancer risk among individuals with one or more of the variant alleles was observed among light smokers (defined as having smoked < or = 30 pack-years). The respective risk ratios for the exon 7 and MspI polymorphisms were 2.26 (95% confidence interval, 0.82-6.26) and 2.03 (95% confidence interval, 1.03-4.01) at low smoking dose. No such increase in risk was found among heavy smokers (> 30 pack-years). This phenomenon at low smoking dose was also observed when the two common polymorphisms were combined, which resulted in was a progressive increase in risk with an increasing number of variant alleles. These results indicate that at low smoking levels, the MspI and exon 7 CYP1A1 genetic polymorphisms confer susceptibility to lung cancer.