Studies on intraabdominal infections have been difficult to compare in the past due to a missing system of classification for peritonitis. According to a recently developed classification system, secondary peritonitis, including spontaneous acute peritonitis, postoperative peritonitis and posttraumatic peritonitis, is the most common complication of severe intraabdominal infections. In several studies the mortality rate of postoperative peritonitis was still between 60% and 79%. Scoring systems were developed, some of them with the idea to predict mortality in peritonitis. Although the APACHE II score cannot predict the outcome of peritonitis in an individual patient, it is a reliable, valid and objective system for risk stratification in intraabdominal infections. Local trauma or bacterial contamination is responsible for an acute phase reaction, which involves the release of certain cytokines such as TNF-alpha, interleukin-1 (IL-1) and interleukin-6 (IL-6). The IL-6 seems to play an important role in the mechanism of the acute phase reaction, acting on hepatocytes to release acute phase proteins (eg CRP). Preliminary results of investigations of IL-6 levels in peritonitis indicate a possible role for IL-6 as a predictor of the outcome of peritonitis.