This is the first observation of a polymorphism associated with the severity of clinical manifestation in Osteogenesis imperfecta (OI). The results are consistent with a polygenic quantitative trait model in which OI albeit a monogenic autosomal dominant disease can be modified by additional risk factors. In our collection the severe cases of OI are significantly associated with a COL1A1 Sp1 polymorphism, an association initially observed in postmenopausal women with osteoporosis. In both studies a G → T polymorphism in the same Sp1 motif is found rather in the clinically severe cases. As diverse vitamin D receptor (VDR)-polymorphisms are one of the strongest risk factors of osteoporosis this prompted us to examine a putative association in our OI collection. However, we could not find a correlation of the severity of OI with any of the four polymorphisms (BsmI, ApaI, and TaqI RFLPs). We conclude that the COL1A1 Sp1 polymorphism is of high prognostic value in OI individuals homozygous for the T allele.