The diversity of biological effects resulting from exposure to dioxin may reflect the ability of this environmental pollutant to alter gene expression by binding to the arylhydrocarbon receptor (AHR) gene and related genes. AHR function may be regulated by structural variations in AHR itself, in the AHR repressor (AHRR), in the AHR nuclear translocator (ARNT), or in AHR target molecules such as cytochrome P-4501A1 (CYP1A1) and glutathione S-transferase. Analysis of the genomic organization of AHRR revealed an open reading frame consisting of a 2094-bp mRNA encoded by ten exons. We found one novel polymorphism, a substitution of Ala by Pro at codon 185 (GCC to CCC), in exon 5 of the AHRR gene; among 108 healthy unrelated Japanese women, genotypes Ala/Ala, Ala/Pro, and Pro/Pro were represented, respectively, by 20 (18.5%), 49 (45.4%), and 39 (36.1%) individuals. We did not detect previously published polymorphisms of ARNT (D511N) or the CYP1A1 promoter (G-469A and C-459T) in our subjects, suggesting that these polymorphisms are rare in the Japanese population. No association was found between uterine endometriosis and any polymorphisms in the AHRR, AHR, ARNT, or CYP1A1 genes analyzed in the present study.