Ehlers-Danlos syndrome (EDS) type IV is a clinically and genetically heterogeneous disorder characterized by thin skin, prominent venous vascular markings, markedly increased bruising, and an increased likelihood of large bowel and large artery rupture. We studied two type IV EDS patients. Both have decreased amounts of type IIII collagen in skin, but ultrastructural examination of dermis showed massive dilation of rough endoplasmic reticulum in dermal fibroblasts in one, but not the other. Both had a major population of collagen fibrils of small diameter. Although previous studies suggested absent synthesis of type III collagen as the hallmark of one type of EDS IV, several abnormalities in metabolism of that type of collagen may be responsible for the phenotype in these disorders. Such disorders are likely to provide better understanding of the function of specific collagens in tissues.