Summary: One thousand seven hundred seventeen perimenopausal women from the Danish Osteoporosis Prevention Study were genotyped for the -1997G/T, -1663indelT and +1245G/T polymorphisms in the COLIA1 gen. We found that the -1997T allele and a haplotype containing it were associated with reduced bone mineral density (BMD) and increased bone turnover at menopause and after 10 years of follow-up. Introduction: We wanted to investigate whether the -1997G/T, -1663indelT and +1245G/T polymorphisms in the COLIA1 gene are associated with perimenopausal bone mass, early postmenopausal bone loss and interact with hormone treatment. Methods: One thousand seven hundred seventeen perimenopausal women from the Danish Osteoporosis Prevention Study were genotyped, and haplotypes were determined. BMD was examined by dual X-ray absorptiometry. Results: Women carrying the -1997T variant had lower BMD at all measured sites: lumbar spine BMD 1.030±0.137 g/cm 2, 1.016±0.147 g/cm 2 and 0.988±0.124 g/cm 2 in women with the GG, GT and TT genotypes, respectively (p<0.05) and total hip BMD 0.921±0.116 g/cm 2, 0.904±0.123 g/cm 2 and 0.887±0.109 g/cm 2 in women with the GG, GT and TT genotypes, respectively (p=0.01). The effect remained after 10 years although statistical significance was lost. Haplotype 3 (-1997T-1663ins+1245G) was associated with lower bone mass and higher levels of bone turnover. Compared with haplotype 1, haplotype 3 carriers had lower BMD at the lumbar spine, femoral neck and total hip by 0.016±0.007 g/cm 2, 0.015±0.006 g/cm 2 and 0.017±0.006 g/cm 2, respectively (p<0.05-0.005). No association with postmenopausal changes in bone mass and fracture risk and no overall interaction with the effects of hormone therapy could be demonstrated for any of the polymorphisms in COLIA1. Conclusions: The -1997G/T polymorphism and haplotype 3 are significantly associated with perimenopausal bone mass, and these effects were sustained up to 10 years after menopause. No association between the -1663indelT or +1245G/T polymorphisms and peri- or postmenopausal bone mass could be demonstrated.