Four patients with paraproteinemia and scleredema were studied. Histologic features included marked thickening and fibrosis of the dermis and subcutis. Variable amounts of mucin deposits were detected in the interfibrillar spaces. Serum from one patient significantly stimulated collagen production in normal skin fibroblast cultures, whereas serum from another patient stimulated collagen production in autologous cell cultures. Moreover, serum from one patient stimulated the [35S] sulfate incorporation into the fibroblasts. Circulating serum factors, possibly related to the paraprotein, may enhance the synthesis of extracellular macromolecules by dermal fibroblasts in these patients, thus providing a mechanism for dermal fibrosis.