In this study of 87 lung cancer patients, 23 patients with lung disease other than cancer, and 121 healthy controls, no association was found between the MspI restriction fragment length polymorphism (RFLP) of the CYP1A1 gene and lung cancer risk. In the lung cancer population, histological type, smoking, and occupational histories were also examined with respect to increased lung cancer risk. No association was found between the MspI RFLP in the CYP1A1 gene and any of these variables. This is in contrast to the results of an earlier report describing an association between the rare genotype m2m2 and susceptibility to lung cancer in a Japanese population; but another study in Norway found no such association. It is evident that, in the Nordic population, MspI polymorphism in the CYP1A1 gene does not indicate individual susceptibility to lung cancer. We also studied a new point mutation which has recently been closely linked to the MspI restriction site polymorphism in a Japanese study population. This mutation results in an isoleucine-valine amino acid replacement in the heme binding region of human CYP1A1. We obtained a similar linkage in our study, so the discrepancy between the Japanese and the Nordic MspI RFLP findings cannot be based on a different degree of linkage between these two point mutations.