The aim of the study was to elucidate the involvement of Fas antigen in human autoimmune disease, by analysing serum levels of soluble Fas/APO-1 protein in patients with various autoimmune diseases, including system lupus erythematosus (SLE), rheumatoid arthritis (RA), systemic sclerosis (SSc), polymyositis/dermatomyositis (PM/DM), Behçet's syndrome and Sjögren's syndrome (SjS). The levels of soluble Fas/APO-1 in sera were quantitated by a sandwich enzyme-linked immunosorbent assay. Soluble Fas/APO-1 levels were significantly increased in serum from patients with primary Sjögren's syndrome (primary SjS) compared with control subjects. However, no significant differences in soluble Fas/APO-1 levels were noted in patients with secondary Sjögren's syndrome (secondary SjS) nor in patients with any of the other autoimmune diseases. The soluble Fas/APO-1 level in primary SjS patients with extraglandular diseases was significantly higher than that in patients without extraglandular diseases. These results suggest that soluble Fas/APO-1 protein may play an important role in the pathogenesis of primary SS.