Specimens of epiphyseal cartilage from ten patients with different types of dwarfism were studied and only in the specimens from patients with pseudoachondroplastic dwarfism were the findings indicative of a specific ultrastructural defect—a storage in the rough-surfaced endoplasmic reticulum of inclusions and whorls with alternately electron-dense and electron-lucent layers. The authors believe the material may be abnormal protein, perhaps lipoprotein or glycoprotein.