Two maternal cousins affected by the X-linked form of Ehlers-Danlos syndrome have been observed. Both had congenital heart disease, "floppy valve syndrome", hernias, short stature, stretchable skin and moderate joint hypermobility. Both excreted normal amounts of urinary glycosaminoglycans, almost entirely represented by dermatan sulfate, whose degradation appeared to be inadequate. They also excreted large amounts of hydroxylysine glycosides and L-valyl-proline, considered to be products of degradation of collagen and elastin, respectively. Cultured skin fibroblasts of the propositus synthesized excessively soluble collagen and had a low lysyl oxidase activity. These findings suggest that the increased degradation of structural proteins may be secondary to the defective cross-linking processes caused by the enzymic defect. Addition of (+) catechin, a flavonoid, to the propositus's cultured fibroblasts decreased the abnormal solubility of their collagen.