A polymorphism in an Sp1 site in the collagen Iα1 (COLIA1) gene has recently been identified and the Ss and ss genotypes were shown to be potentially important determinants of low bone mass in postmenopausal women. Additionally, in a Dutch population, the association of the COLIA1 polymorphism with low bone mineral density (BMD) was more pronounced with increasing age, suggesting a genotype effect on the rate of bone loss. We have investigated the relationship between the COLIA1 Sp1 polymorphism and the rate of bone loss in a longitudinal study with a total of 133 postmenopausal women followed for 18 years. The frequencies of the genotypes were SS 70.7%, Ss 27.8%, ss 1.5% and were in Hardy-Weinberg equilibrium. No association of the COLIA1 genotype with rate of bone loss was detected and there was no difference between the genotype groups with respect to BMD at the femoral neck or lumbar spine. Women with the Ss or ss genotypes, who have been postulated to have low BMD, had even higher BMD at the lower forearm than women with the SS genotype. The levels of serum osteocalcin and urinary collagen type I degradation products were not found to be associated with the COLIA1 Sp1 polymorphism. In conclusion, this study does not support the hypothesis that the Ss COLIA1 genotype predisposes women to increased rate of bone loss or low BMD. However, because of a low absolute number of the ss genotype, it was not possible to reach a conclusion on this particular genotype with regard to an association with low BMD or rate of bone loss.