To elucidate the mechanisms involved in the development of cutaneous fibrosis in scleredema adultorum, we studied a patient with long-standing scleredema who had no history of diabetes mellitus or preceding febrile illness. Histological examination of a biopsy specimen from involved forearm skin demonstrated marked thickening of the dermis and accumulation of mucin between collagen bundles. Increased levels of type I collagen mRNA, as evidenced by positive in situ hybridization signals with an alpha 1(I) procollagen cDNA were found in numerous fibroblasts throughout the dermis. The expression of several genes coding for proteins involved in the maintenance of connective tissue was examined by determining in vitro protein production and mRNA levels in fibroblasts from the affected skin. Total protein production, glucosamine incorporation and collagen synthesis, were elevated by 44-97% in scleredema fibroblasts, compared with fibroblasts from two healthy individuals. Levels of mRNAs for alpha 1(I) and alpha 1(III) procollagens and fibronectin were elevated in scleredema fibroblasts, whereas mRNA levels for the tissue inhibitor of metalloproteinase were unaltered compared with control cultures. The results suggest that fibroblasts from the involved skin in non-diabetic patients with scleredema may exhibit a biosynthetically activated phenotype, which persists for several years. These alterations are likely to be involved in the development of the cutaneous induration and thickening which is characteristic of this disease.