We have previously reported on an association of genetic susceptibility to squamous cell carcinoma of the lung with two polymorphisms of the CYP1A1 gene, an MspI polymorphism and an isoleucine-valine (Ile-Val) polymorphism. We report here that the two CYP1A1 polymorphisms were associated with current or ex-smokers with adenocarcinoma of the lung. We first compared smoking status of the patients by differentiation grades of adenocarcinoma. Proportion of current or ex-smokers and their cigarette consumption was the highest among the poorly differentiated. Second, the two polymorphisms of CYP1A1 were examined among current or ex-smokers with three differentiation grades, and we found that poorly differentiated adenocarcinoma showed significantly high frequencies of genotypes C and Val/Val, which have been found to be 'susceptible' in squamous cell carcinoma of the lung. Third, a case-control study was carried out to estimate the genetic risk for CYP1A1 genotypes to differentiation grades, selecting only current or ex-smokers in patients and controls. Current or ex-smokers with 'susceptible' genotype C or Val/Val were at significant risk, with an odds ratio of 3.25 or 3.22 (95% confidence interval (CI), 1.40-7.56 or 1.00-10.32) for adenocarcinoma as a whole, respectively. The odds ratio increased to 4.51 or 4.09 (95% CI, 1.73-11.78 or 1.12-14.91) for poorly differentiated, while the odds ratios for other differentiated grades were not significant. Finally, a relation of the genetic risk to cigarette dose levels was evaluated.