The soluble form of Fas (sFas) can block apoptosis induced by the Fas ligand in vitro. A recent report demonstrated that mice injected with sFas displayed autoimmune features. Therefore, an elevated serum concentration of sFas may be associated with lymphoproliferation and autoimmune diseases. We measured the serum concentrations of sFas in 77 patients with non-Hodgkin's lymphoma (NHL) [8 angioimmunoblastic T-cell lymphoma (AIL), 12 T-cell NHL, 53 B-cell NHL, and 4 natural killer-cell NHL]. Elevated concentrations of sFas were detected only in AIL, which is frequently accompanied by autoimmune diseases (P < 0.005 compared with age-matched controls). A possible association of sFas and autoimmune features in AIL is discussed.