Polymorphism of the gene encoding for debrisoquine hydroxylase, i.e. CYP2D6, was determined genotypically for 122 healthy controls and 106 lung cancer patients using Xba I restriction fragment length polymorphism (RFLP) analysis, together with a combination of two recently published polymerase chain reaction (PCR) based approaches. Three different mutated alleles of the CYP2D6 gene were detected; CYP2D6B comprised 11.1% and 10.4% of the total alleles in the controls and in the lung cancer patients, CYP2D6A had frequencies of 5.7% and 2.8%, and CYP2D6D had frequencies of 3.3% and 2.4%, respectively. Only 17 of the 24 44 kb Xba I alleles (71%) were confirmed as defective alleles carrying the mutation in CYP2D6B loci, whereas all four 15 + 9 kb Xba I alleles contained the CYP2D6B mutation. Out of the 122 healthy controls, seven subjects (5.7%) were detected as poor metabolizers (PMs) of debrisoquine by the presence of two defective alleles, whereas only one PM genotype was found in the lung cancer patient group (0.9%). The reliability of this analysis was confirmed in a subgroup of the control subjects phenotyped by debrisoquine, where a perfect correlation between CYP2D6 phenotype and genotype was obtained. We observed no significant difference in the allelic frequencies between lung cancer patients with a history of heavy smoking and those who smoked less. However, statistical analysis showed a significant difference (p = 0.05) in distribution of the PM-associated genotypes between lung cancer patients (1/106) and healthy controls (7/122). This data thus supports the hypothesis that there is an increased risk of lung cancer for individuals who are extensive metabolizers of debrisoquine.