Osteoporosis is a common disease with a strong genetic component^1–3, characterized by reduced bone mass and increased fracture risk⁴. Current evidence suggests that the inheritance of bone mass is under polygenic control⁵ but the genes responsible are poorly defined. Type I collagen is the major protein of bone encoded by the COLIA1 and COLIA2 genes. While these are strong candidates for the genetic regulation of bone mass, no abnormality of either gene has so far been defined in osteoporosis⁶. In this study, we describe a novel G→T polymorphism in a regulatory region of COLIA1 at a recognition site for the transcription factor Sp1⁷ that is significantly related to bone mass and osteoporotic fracture. G/T heterozygotes at the polymorphic Sp1 site (Ss) had significantly lower bone mineral density (BMD) than G/G homozygotes (SS) in two populations of British women and BMD was lower still in T/T homozygotes (ss). The unfavourable Ss and ss genotypes were over-represented in patients with severe osteoporosis and vertebral fractures (54%), as compared with controls (27%), equivalent to a relative risk of 2.97 (95% confidence interval 1.63–9.56) for vertebral fracture in individuals who carry the ‘s’ allele. While the mechanisms that underlie this association remain to be defined, the COLIA1 Sp1 polymorphism appears to be an important marker for low bone mass and vertebral fracture, raising the possibility that genotyping at this site may be of value in identifying women who are at risk of osteoporosis.