The authors report six cases from six different families of lethal brittle bone disease with narrow diaphyses and thin ribs. This phenotype should be dissociated from the lethal forms of osteogenesis imperfecta and encompass two diseases. In the first, autosomal recessive, the metaphyses of long bones are narrow, with a membranous ossification, without cartilagenous residue. Cultured fibroblasts demonstrate a marked increase in type V collagen. In the second type, the metaphyses are enlarged and the babies have a facial dysmorphism with hypoplasia of the eyebrows, frontal bossing and a small mouth.