This report deals with an unusual case of spondyloepiphyseal dysplasia, where the diagnosis was suspected clinically and radiologically and where cartilage and osseocartilaginous specimens were taken for ultrastructural and biochemical analyses. Unlike cases described previously the electron microscopic studies of chondrocytes showed dilatation of the rough endoplasmic reticulum cisternae. The dilated cisternae contained an amorphous substance, and the picture is suggestive of a metabolic disturbance. The results of the biochemical analyses were within the normal range and did not indicate storage of glycosaminoglycans.