Progressive systemic (sclerosis) is one of the most enigmatic of the rheumatic diseases. It is a connective tissue disorder of unknown etiology characterized by fibrosis in skin and internal organs. Although similar lesions are found with increased prevalence in workers exposed to coal, gold, silica, and polyvinyl chloride, most patients have had no known predisposing factors. Select reports of a familial occurrence of PSS have been observed but a definitive genetic basis is lacking and no clear associations with the major histocompatability complex have been demonstrated. Moreover, although a variety of immunologic abnormalities in patients with PSS have been reported, they are generally diffuse and non-diagnostic. Such abnormalities include defects in cell mediated immunity, increases in sera immunoglobulins, antinuclear antibodies, and cryoglobulins. In contrast to these non-specific findings, there appears to be significant evidence of a relationship between cell mediated immunity to collagen and appearance of scleroderma. For example, peripheral blood lymphocytes in patients with scleroderma undergo lymphocyte transformation when cultured with specific collagen preparations. The pathology of skin and internal organs in PSS generally reflects both collagen deposition and small vessel occlusion. All organ systems may be involved but mortality significantly increases with involvement of heart, kidney, or lung. Unfortunately, at present a reliable experimental model of PSS has not been found although similar immunopathology can be induced in homologous disease of rats and in chronic graft vs host disease of humans.