Chromosomal mapping of the human estrogen receptor beta (ERP) gene by fluorescence in situ hybridization (FISH) reveals that ERP is located at human chromosome 14, region q23-24.1, where the aberration of DNA copy number in the bone disorders is frequently involved. Then, me investigated the association between dinucleotide (cytosine-adenine; CA) repeat polymorphism located in the flanking region of ERP gene and bone mineral density (BMD) in 204 healthy postmenopausal Japanese women. The genotype was classified into "A" through "O" according to the number of the repeats, from 18 to 32. BMD was expressed in Z score (a deviation from the weight-adjusted average BMD of each age using the standard deviation as a unit). When we separate the subjects into two groups bearing at least one I allele (26 CA repeats) and those who did not, the former subjects had significantly higher Z score of L2-4 BMD (mean +/- standard deviation; 0.674 +/- 1.53 vs 0.128 +/- 1.38; P = 0.027). These data suggest that genetic variation at the ERP locus may be associated with some determinants for BMD and the possible involvements of this polymorphism in the cause of postmenopausal osteoporosis in Japanese women.