The polymorphism of insulin-like growth factor-I (IGF-I) gene was examined in Japanese postmenopausal women to analyze the genetic background for osteoporosis. In this study, the dinucleotide (cytosine-adenine; CA) repeat sequence lying upstream of the transcription initiation site of this gene was examined. We named the most frequent allele including (CA) 19 as J allele. There were 6 alleles (J-4 containing 17 CA repeats: (CA)17, [J-2 (CA)18, J (CA)19, J + 2 (CA)20, J + 4 (CA)21, J + 6 (CA)22]) in the Japanese population. The genotype distribution was different from that of Caucasians. There was no different in bone mineral density (BMD) between the group with one or two alleles of each genotype and that without that genotype. When we separate the subjects into three groups having two alleles, one allele, and no alleles, the three subjects who possess the allele 'J-2' in both strands had low BMD (Z score of L2-4; -1.24 +/- 0.56, total body; -0.943 +/- 0.59, mean +/- SE). On the other hand, sequence of IGF-I gene in this study was different from reported sequence of IGF-I gene; that was 2 base pair (bp) deletion following 3'end of CArepeat (-645adenine/-646guanine). The present study showed that there was no association between the microsatellite polymorphism of IGF-I gene and BMD in Japanese postmenopausal women, but some possibility remains that the microsatellite polymorphism of IGF-I gene is useful to detect a kind of particular osteoporosis.