Recent biochemical, linkage, and molecular genetic studies have demonstrated that, in almost every instance, osteogenesis imperfecta results from mutations in the genes that encode the chains of type I collagen. Such studies have done much to improve our understanding of the molecular basis of brittle bone disease, and have provided significant inroads into molecular diagnosis and prognostic counseling. Nonetheless, further investigation is needed urgently to identify forms of medical therapy that will decrease morbidity in osteogenesis imperfecta.