In a girl followed from early childhood to maturity because of clinical, laboratory and roentgenographic findings typical of metaphyseal dysostosis, light and electron microscopic studies of cartilage from the ulnar epiphyseal plate and iliac crest obtained at eight and eleven years of age revealed the cisternae of the chondrocytes' endoplasmic reticulum to be markedly dilated by a granular precipitate. It was concluded that the specific defect in this disease results in storage of an undetermined substance in the rough-surfaced endoplasmic reticulum.