The FasL-Fas system has been recognized as one of the apoptosis-inducing systems in the body. We studied the association between the serum levels of sFas, sFasL and TNF-alpha and the severity of the pathophysiological condition in 20 patients with multiple organ dysfunction syndrome (MODS) complicating generalized peritonitis. The serum levels of sFas and TNF-alpha were significantly higher in the patients who died than in those who survived. On the other hand, the sFasL level was significantly higher in the patients who survived than in those who died. A significant correlation was observed between the serum TNF-alpha level and the serum sFas level. These findings suggest that changes in the FasL-Fas system may be involved in the pathogenesis of MODS.