Osteogenesis imperfecta (OI) Type II is a rare heritable disorder of bone matrix that results in catastrophic congenital skeletal dysplasia. Two cases of OI Type II had symmetric rhizomelic skeletal dysplasia apparent on ultrasound at 16 and 20 weeks' gestation. Histologic and histochemical studies performed on skeletal tissue from fetal autopsies showed the following: (1) abnormal growth plate tissue characterized by failure of formation of primary bony spongiosa; (2) persistence of calcified cartilage bars in the diaphysis; (3) metaphyseal microfractures; (4) abundant cartilaginous fracture callus; (5) absence of bony callus; (6) failure of formation of intramembranous cortical diaphyseal bone; (7) angulation of long bones in portions of the metadiaphyses bordered by fracture callus; and (8) mechanical failure of the perichondral ring of LaCroix with a normal fibrous ossification groove of Ranvier. These findings suggest that skeletal dysplasia in OI Type II results from the action of muscular forces on a skeleton weakened by a complex disorder of endochondral and intramembranous ossification. The paucity of primary metaphyseal trabeculae and subperiosteal cortical bone leads to pathologic fractures of the immature fiber bone and an imperfect attempt at fracture repair. Angulation and shortening of long bones occurs between numerous sites of focal endochondral fracture callus. Mechanical failure of the fibrous perichondral ring leads to further collapse and shortening without obvious functional impairment of the fibrous ossification groove. Perinatal lethal OI provides insight into how a molecular disorder predominantly of Type I collagen metabolism results in pathology of numerous tissues, leading to severe skeletal dysplasia without primarily affecting chondrogenesis.