BACKGROUND: Genetic variation of CYP2A6 is shown to alter nicotine metabolism. This study was developed to investigate the genetic influence of the whole deletion-allele of CYP2A6 on active and passive smoking behavior. METHODS: Two hundred and forty Japanese adults, who visited Aichi Cancer Center as outpatients, were genotyped for the wild-type (CYP2A6∗ 1A, CYP2A6∗ 1B) and the whole deletion-type (CYP2A6∗4C) polymorphism of CYP2A6. Information about active and passive smoking status was obtained by a self-administered questionnaire. Genetic influence of CYP2A6 polymorphism on smoking behavior was evaluated using the Mantel extension test. RESULTS: The frequency of the deletion allele was 18%. All 8 subjects carrying two deletion alleles had no smoking habit, and the homozygous deletion genotype showed a tendency to correlate with active smoking status after adjustment for sex and age (p=0.054). However, the proportion of never smokers among heterozygous subjects was almost the same as among subjects carrying no deletion allele (54% and 58%, respectively). Furthermore, CYP2A6 genotypes were correlated neither with the number of cigarettes smoked per day nor with the age at starting smoking (p=0.364 and 0.880, respectively). Among never smokers, CYP2A6 genotypes were not correlated with exposure to passive smoking at home or in the workplace (p=0.623 and 0.484, respectively). CONCLUSION: Despite the possible protection against active smoking behavior in subjects homozygous for the deletion allele, the CYP2A6 polymorphism has only a limited impact on public health because no protective effect was found in heterozygous subjects. J Epidemiol2003;13:176-181.