Summary Osteogenesis imperfecta, defective bone development, and dentinogenesis imperfecta, defective tooth development, are familial diseases due to mesenchymal hypoplasia. These mesodermal disturbances appear to be genetically dominant but variable in the degree of expression. Either one or both defects may be present in the same kinship or the same immediate family; or both may occur in any one individual member. It is the general concensus that these two clinical entities are manifestations of the same basic pathologic state. The present report is based on a study of 18 families in which a total of 167 individual members showed one or both defects. With one possible exception, no consanguinity was known in these 18 families.