Autosomal dominant inheritance of a mild form of osteogenesis imperfecta (osteogenesis imperfecta type I) with different phenotypic expression was found in a family. Phenotypic expression was different for the affected mother and son, in the presence of the same biochemical results. Dermal fibroblast cultures synthesized normal and mutant type I collagen alpha chains. Collagen heterotrimers containing abnormal chains were overmodified along the entire triple helical domain and showed an unusually low denaturation temperature, so far found only in lethal cases. The mild phenotype in the family is probably due to the fact that abnormal type I collagen molecules are more likely to be degraded than utilized in the extracellular matrix.