OBJECTIVE Experimental studies have suggested that apoptosis via the Fas/Fas Ligand signaling system may play an important role in the development of acute liver failure. The aim of the study was to investigate the soluble form of Fas in patients with acute liver failure. METHODS Serum levels of sFas (soluble Fas) were measured by ELISA in 24 patients with acute liver failure and 10 normal control subjects. Serum levels of tumor necrosis factor-alpha and interferon-gamma were also determined by ELISA. RESULTS Serum sFas was significantly increased in patients with acute liver failure (median, 26.8 U/mL; range, 6.9-52.7 U/mL) compared to the normal controls (median, 8.6 U/mL; range, 6.5-12.0 U/mL, P < 0.0001). Levels were significantly greater in patients with acute liver failure due to paracetamol overdose (median, 28.7 U/mL; range, 12.8-52.7 U/mL, n = 17) than those due to non-A to E hepatitis (median, 12.5 U/mL; range, 6.9-46.0 U/mL, n = 7, P < 0.01). There was no relationship of sFas to eventual outcome in the patients. A significant correlation was observed between serum sFas levels and aspartate aminotransferase (r = 0.613, P < 0.01). CONCLUSIONS The increased concentration of sFas in serum of patients with acute liver failure may reflect activation of Fas-mediated apoptosis in the liver and this together with increased tumor necrosis factor-alpha may be an important factor in liver cell loss.