Spontaneous ascites infection (SAI) is an important complication of decompensated cirrhosis. Its prototype form is spontaneous bacterial peritonitis (SBP). We investigated the value of determining ascitic fluid interleukin-6 (IL-6) levels in the diagnosis of SEP and in monitoring responses to therapy. The study group consisted of 28 patients with liver cirrhosis and ascites. We diagnosed SEP in 16 patients. Ascitic fluid IL-6 levels were significantly higher in patients with SEP than in those without this condition (1957 +/- 65 pg/dl and 871 +/- 42 pg/dl, respectively P < 0.0001). Ascitic fluid IL-6 levels declined after treatment (1377 +/- 85 pg/dl, P < 0.01). These findings suggest an increase in local IL-6 synthesis in the peritoneal cavity. We conclude that ascitic fluid IL-6 levels may be a useful tool for the diagnosis and monitoring of SEP. Med Sci Res 27:167-168 (C) 1999 Lippincott Williams & Wilkins.