The mitochondrial DNA (mtDNA) of hair follicles was a mild increase in the percentage of the 7,436 was used for studying the genotoxicity of smoking-bp dmtDNA with the increase of the consumption mediated carcinogens. We determined the inci-of cigarettes. No tandem duplication of mtDNA in dences of the 4,977 bp and 7,436 bp mtDNA dele-the D-loop region was disclosed in either smokers tions, tandem duplication in the D-loop region and or non-smokers group. The proportions of the 4,977 the proportion of the 4,977 bp deleted mtDNA bp dmtDNA in hair follicles were found to correlate (dmtDNA) in the total DNA of hair follicles from with age, but did not keep increasing with cigarette 213 male non-smokers and 74 male smokers, re-consumption except in the group of subjects with a spectively. Twenty-three patients with lung cancer smoking index of less than 400. On the other hand, were also investigated. We found that the current we found that the average proportion of the 4,977 cigarette smokers had a 3.1 times higher average bp dmtDNA in the hair follicles was 1.201 { incidence of the 4,977 bp dmtDNA (RR: 3.1, P รต 0.371% for the patients with lung cancer who had 0.001) as compared with non-smokers, and this a smoking index greater than 400, while that was mtDNA deletion was especially prevalent in the old only 0.146% for the age-matched healthy smokers heavy smokers. For the smokers of the age above with the same smoking index. In conclusion, the 70, the average incidence of the 4,977 bp high incidence of the 4,977 bp dmtDNA of hair dmtDNA was 3.7 times higher in the group with a follicles is not only associated with aging but also smoking index of 401-800 (RR: 3.7, P รต 0.005) correlated with the amount of cigarette smoking. A and 3.2 times higher in the group with a smoking high proportion of the 4,977 bp dmtDNA in the index greater than 800 (RR: 3.2, P รต 0.005). How-hair follicles may be considered one of the molecuever, there was no statistically significant relation-lar events that are associated with the occurrence ship between the incidence of the 7,436 bp of smoking-associated cancers.