Objectives. To develop a new system for predicting the outcome of secondary peritonitis and analyze its accuracy in comparison with the most common analogous systems. Methods. The study is based on the analysis of treatment results in patients (n=352) with secondary peritonitis. At admission sepsis was diagnosed in 15 (4.3%) patients, septic shock – in 4 (1.1%) persons. There were the following main causes of death in the mortality structure: purulent intoxication and/or sepsis – 51 cases (87.9%), cancer intoxication – 4 (6.9%) cases, acute cardiovascular failure – 3 cases (5.2%). The efficacy of the Mantheim Peritoneal Index (MPI), WSES prognostic score, APACHE-II scale, gSOFA score and Peritonitis Prediction System (PPS) developed by the authors were analyzed. The likelihood of the effect of 85 clinical and laboratory parameters on the outcome of patients with secondary peritonitis using nonparametric methods of statistical research (Fisher's test, Mann-Whitney test, Chi-square with Yates correction) have been analyzed. Criteria predictively associated with lethal outcome (p <0.05) were selected, they were included in the PPS scale. To compare the predictive value of peritonitis prediction systems, ROC analysis was used with the construction of ROC curves for each of the systems. Results. The most important criteria in predicting fatal outcome are the patient’s age, the presence of malignant tumor, the exudate nature, sepsis (septic shock), and also polyorganic insufficiency which is not associated with developed peritonitis. To assess the prognostic value of peritonitis prediction systems, ROC curve analysis was used. The greatest accuracy in terms of predicting mortality in patients with generalized secondary peritonitis is possessed by PPS (AUC 0.942), minimal – APACHEII (AUC 0.840). Conclusion. APACHEII, MPI, WSESSSS and PPS systems can be considered as reliable in predicting mortality in patients with peritonitis. The greatest accuracy in predicting fatal outcome in patients with generalized secondary peritonitis had PPS (94%). © 2021 Vitebsk State Medical University. All rights reserved.