Given that osteoprotegerin plays an important role in bone remodeling, the osteoprotegerin gene may be a candidate locus for susceptibility to osteoporosis. The relation of polymorphisms in the promoter of the osteoprotegerin gene to bone mineral density (BMD) was examined in a Japanese population-based prospective cohort study with randomly recruited subjects (1095 women and 1125 men for the 950T --> C polymorphism, 1094 women and 1127 men for the 245T --> G polymorphism). BMD at the radius was measured by peripheral quantitative computed tomography, and that for the total body, lumbar spine, right femoral neck, right trochanter, and right Ward's triangle was measured by dual-energy X-ray absorptiometry. Genotypes were determined with a fluorescence-based allele-specific DNA primer assay system. Among 950T --> C genotypes, BMD for the proximal radius was lower in premenopausal women with the CC genotype than in those with the TT or TC genotype; the difference in BMD between the two groups was 3.9% (P=0.0075). Among 245T --> G genotypes, BMD for the radius, total body, femoral neck, trochanter, and Ward's triangle was lower in postmenopausal women with the GG genotype than in those with the TT or TG genotype, the TT genotype, or the TG genotype; the differences in BMD between the GG genotype and the TT or TG genotype were 19.8% for the distal radius (P=0.0015), 13.1% for the proximal radius (P=0.0095), 11.2% for the total body (P=0.0013), 12.9% for the femoral neck (P=0.0067), 18.7% for the trochanter (P=0.0008), and 27.1% for Ward's triangle (P=0.0038). BMD was not associated with the 950T --> C or 245T --> G genotypes in men. The present results implicate the osteoprotegerin gene as a susceptibility locus for reduced BMD in Japanese women.