The mechanism responsible for prenatal bowing of long bones was investigated based on the results of morphological observation and biochemical analysis of glycosaminoglycans of the autopsied bone from a newborn patient with classic campomelic dysplasia and the result of an experiment which produced a similar deformity in rat fetuses. An excised bone with postnatal bowing from a patient with osteogenesis imperfecta was also examined for comparison. In the case of campomelic dysplasia, marked hypertrophic change of the cambium layer of the periosteum at the concave side of the bone was noted. Parallel rays of the periosteal bone extended from the concave (posterior) side toward the apex of the bowing at the anterior side. Increased amounts and differences in composition of glycosaminoglycans were found in the diaphyseal bone on the concave side, suggesting the existence of bone with maturity retardation. In contrast, there were no such differences in the case with postnatal bowing and in the control. Congenital bowing of long bone in the patients with multiple malformations such as campomelic dysplasia is probably manifested as early as the cartilaginous model as the experimental result suggested.