INTRODUCTION: Peritonitis is a life-threatening disease with high mortality and morbidity. The prognosis depends on patient factors, the nature of the disease, and on diagnostic and therapeutic methods. The goal of our study was to perform a retrospective analysis of a group of patients with secondary peritonitis and to compare patients with non-stercoral peritonitis and those with stercoral peritonitis, and finally, to determine the prognostic value of the MPI prognostic index. METHODS: We analysed 124 patients who had undergone surgery for secondary peritonitis during the years 2012-2013. We divided the patients into two groups. Group A comprised patients with non-stercoral peritonitis and group B consisted of patients with stercoral peritonitis. We compared the two groups and predicted the peritonitis outcome using the Mannheim Peritonitis Index (MPI). RESULTS: The complete sample of 124 patients consisted of 70 men and 54 women. The average age of the patients was 63 years and the average length of hospitalization was 18 days. In total, 18 patients (15%) died during their hospitalization. The average MPI score was 19, which correlates to an 18% mortality rate prediction. In group A, which consisted of 68 patients with non-stercoral peritonitis, the average age was 59 years and the average length of hospitalization was 12 days. Two patients (3%) from this group died during hospitalization. The average MPI score was 9, which correlates to a 9% mortality rate prediction. In group B, which consisted of 56 patients with stercoral peritonitis, the average age was 67 years and the average length of hospitalization was 25 days. 16 patients (29%) from this group died during hospitalization. The average MPI score was 29, which correlates to a 31% mortality rate prediction. CONCLUSIONS: The outcome of our comparison between the two groups is that group A with non-stercoral peritonitis had a significantly lower mortality, lower number of complications and a shorter length of hospitalization. Both basic clinical data and sophisticated scoring systems can be used for mortality prediction in peritonitis. The Mannheim Peritonitis Index, a simple scoring system, proved to be useful in our study.