Skin from four patients with pseudoxanthoma elasticum is studied histochemically using stains for elastic tissue, calcium, and acid mucopolysaccharides. Skin from one of these individuals is studied biochemically with quantitation of total hexosamine, amino sugars, calcium, nonfibrous and fibrous proteins. Histologically and biochemically there is evidence of increased acid mucopolysaccharides which are hyaluronidase resistant and may be sulfated. It is postulated that the increased calcification seen in the lesions is related to this increased acid mucopolysaccharide. There is a reduction of collagen which appears to be primarily in the insoluble collagen fraction. The increased elastin-like protein isolated from pseudoxanthoma elasticum has an amino acid composition which is quite similar to normal human dermal elastin.