NAT1, which biotransforms many carcinogens, is genetically polymorphic. This polymorphism has been postulated as a mechanism for susceptibility differences in cancer, possibly due to NAT1 activity differences. However, the relationship betweenNAT1genotype and phenotype is not clear. In our study of 110 Japanese, the frequency of theNAT1*10allele (0.53, 95% confidence interval 0.46-0.59) was higher than others have observed in Caucasians (0.16). From genotype frequency studies, 26.4% of the subjects belonged to theNAT1*10/*10genotype, 53.6% to theNAT1*4/*10genotype and 20% to theNAT1*4/*4genotype. NeitherNAT1*3norNAT1*11genotype was seen in these subjects. In female subjects, we found higher NAT1 activity inNAT1*4/*10subjects than inNAT1*4/*4subjects (n= 49; 2.63 versus 2.16 nmol/min/mg protein). NAT1 activity-difference betweenNAT1*4/*10andNAT1*10/*10was not statistically significant. Thus, not only the presence ofNAT1*10allele, but also other factors are suspected of increasing NAT1 activities. After full sequencing of 10 subjects, five individuals having the highest activities and five individuals having the lowest activities, we foundNAT1*18AandNAT1*18Bto be in the high activity group and the low activity group, respectively. The genotypes containing these variants were heterozygous, i.e.NAT1*4/*18AandNAT1*4/*18B.Due to rare frequencies of these variants, they cannot be considered as other effective, genetic factors on NAT1 activity. Age and tobacco smoking did not affect the relationship betweenNAT1genotype and phenotype.