BACKGROUND Early classification of patients presenting with peritonitis and intra-abdominal sepsis by means of objective scoring systems is desirable to select patients for 'aggressive' surgery and to compare results of different treatment regimens. However, none of the existing scoring systems has fulfilled all expectations. METHODS Evaluation of the value of various scoring systems (Acute Physiology And Chronic Health Evaluation (APACHE) II, Simplified Acute Physiology Score, Sepsis Severity Score, Multiple Organ Failure, Mannheim Peritonitis Index (MPI), Ranson and Imrie) was performed in 50 patients. Additionally, scoring systems were combined to obtain a 'combined score' for the prediction of peritonitis-related in-hospital death. Hazard ratios were calculated in a univariate and multivariate analysis. RESULTS In the univariate analysis all scoring systems, except Ranson and Imrie, predicted the primary outcome. In the multivariate analysis, only the APACHE II score (hazard ratio 6.7) and the MPI (hazard ratio 9.8) contributed independently to the prediction of outcome. All patients with an APACHE II score of 20 or more and a MPI of 27 or more died in hospital. CONCLUSIONS Combination of the APACHE II and the MPI provides the best scoring system fitting clinical goals.