Calcitonin (CT), a calcium-regulating hormone,lowers the calcium level in serum by inhibiting bone resorption.Because CT may play a role in the pathogenesis ofosteoporosis, genetic variations in or adjacent to the CTgene may be associated with variations in bone mineraldensity (BMD). The present study examined the correlationbetween a dinucleotide (cytosine-adenine; CA) repeatpolymorphism at the CT locus and BMD in 311 Japanesepostmenopausal women (mean age, 64.1 years). Seven alleleswere present in this population; each allele contained 10,11, 16, 17, 18, 19, or 20 CA repeats. Thus, we designated therespective genotypes A10, A11, A16, A17, A18, A19, andA20. The A10 and A17 alleles were the predominant allelesin the population studied. Z scores (a parameter representingdeviation from the age-specific weight-adjusted averageBMD) were compared between individuals that possessedone or two alleles of each genotype and those that did notpossess the allele. Subjects who possessed one or two A10alleles had lower BMD Z scores than those who did not(lumbar 2–4 BMD Z score; 20.148 6 1.23 vs 0.182 6 1.54;P 5 0.04). No significant relationships were observedbetween allelic status and background data or biochemicalparameters. The significant association observed betweenBMD and genetic variations at the CT locus implies thatpolymorphism at this locus may be a useful marker for thegenetic study of osteoporosis.