Biochemical examinations of a hyalinized skin tumor of systemic hyalinosis (juvenile hyaline fibromatosis) revealed an increase in chrondroitin sulfate synthesis by skin fibroblasts with an enlarged, rough surfaced endoplasmic reticulum as well as an increase in chondroitin sulfate content. Thse results indicate that systemic hyalinosis is a connective tissue disorder characterized by abnormal GAG synthesis of fibroblasts.