Previous reports of the association of extensive debrisoquine metabolism, controlled by the cytochrome P450 CYP2D6, with increased lung cancer risk have been conflicting. We examined the hypothesis that genetic polymorphism at the CYP2D6 locus identifies individuals at increased risk for lung cancer in a case-control study of 98 incident Caucasian lung cancer patients and 110 age-, race-, and sex-matched controls conducted at the National Naval Medical Center, Bethesda, MD. Using germ line DNA, we identified inactivating mutations at the CYP2D6 locus (CYP2D6*3, CYP2D6*4, CYP2D6*5, and CYP2D6*6A), as well as those mutations that impair but do not abolish enzyme activity (CYP2D6*9 and CYP2D6*10A). Compared to subjects with homozygous inactivating mutations, no association with lung cancer was observed for those with homozygous or heterozygous functional alleles (odds ratios were 0.4 and 0.7, respectively). Furthermore, no excess risk was seen in any histological group or smoking category, and adjustment for smoking and sociodemographic characteristics did not alter the findings. Although the concept that genetic polymorphisms may contribute to differential lung cancer susceptibility is sound, these data do not support the role of CYP2D6 as a marker for elevated lung cancer risk.