Light and electron microscopic studies on cells and matrices of congenital pseudarthrosis of the femur of a 5.5-year-old male revealed three areas with different morphological features; a fibrous area, a cartilaginous area and an osseous area. Glycosaminoglycans were isolated from each of the three areas and characterized by the degradation with chondroitinases. Although chondroitin sulfate was the major component in all regions, a significant amount of dermatan sulfate occurred in the fibrous area. A small amount of hyaluronic acid was found in both fibrous and osseous areas. These suggest that the peculiar composition of glycosaminoglycans in the affected part may account for some clinical aspects of the disease which resists treatment.