Polymorphism at an Sp1 binding site in the COL1A1 gene has been reported to be associated with bone mineral density (BMD) and osteoporotic vertebral fracture. We therefore examined for associations and linkage of the Sp1 polymorphism in the COL1A1 gene and BMD at the lumbar spine and femoral neck in 38 monozygotic (MZ) and 40 dizygotic (DZ) twin pairs of white adult women. All twins were premenopausal with an age range of 21-49 years. Sp1 genotypes of 56 patients with idiopathic osteoporotic vertebral fracture were examined for a preponderance of either genotype relative to our normal healthy twin subjects. In the twin sample no significant association was found between Sp1 genotypes and BMD at the spine and femoral neck. No linkage of Sp1 genotype and BMD at the spine or femoral neck was observed in DZ twins discordant for genotype. Frequencies of Sp1 genotypes were similar in our healthy (twin) and fracture population samples. In conclusion, in our American sample of premenopausal twins we found no association or linkage of the Sp1 polymorphism at the COL1A1 gene and BMD at the lumbar spine and femoral neck, and no over-representation of any Sp1 genotype was observed in our sample of patients with osteoporotic vertebral fracture. Taken together these results indicate that the Sp1 polymorphism is not related to BMD in our American sample, and contrasts with the findings in a British population.