Two male patients (53 and 19 years old) have been seen with a diffuse scleroderma-like illness with firm taut skin bound down to underlying structures (sparing the face in both patients and the hands and feet in one). Flexion contractures of elbows and knees and limitation of abduction at the shoulders developed in a few weeks after onset. Raynaud's phenomenon was absent and thorough evaluation failed to reveal any evidence of myositis or the visceral manifestations of systemic sclerosis. There was no loss of skin appendages. Both had circulating eosinophils (12% and 37%) without drug ingestion or parasitic infestation. Both had elevated sedimentation rates and hypergammaglobulinemia (in one, IgG = 4.1 g.%). Serologic tests for syphilis, rheumatoid factor, LE cells, antinuclear antibodies, complement and cryoglobulins were negative or normal in both patients. Bone marrow examination revealed plasmacytosis and eosinophilia. Biopsies revealed striking thickening of the fascia between the subcutis and muscle. Within the thickened connective tissue there was intense infiltration with plasma cells and lymphocytes, at times in follicles, both about and separate from vessels; eosinophils were absent. Skin biopsies revealed no changes of scleroderma; and muscle biopsies, no evidence of myositis. Prednisone therapy over 15 months in decreasing dosage, using laboratory parameters, induced in the first patient a full remission which has persisted for 5 years without therapy. In the second (more severely involved) patient prednisone therapy has reversed laboratory abnormalities; but clinical improvement to date has been minimal. The pathogenesis of this diffuse fasciitis is obscure, although unusual physical exertion antedated the onset of illness in each case.