Postoperative peritonitis (PoP), despite their relatively low incidence, are associated with high mortality. Such poor outcomes are also related to the high proportion of aged patients, whose intra-abdominal infections are difficult to manage. The study included 84 consecutive patients with PoP. The aim was the validation of the Mannheim Prognostic Index (MPI) in the context of PoP and the assessment of the prognostic impact of age and other clinical factors in a large series from a tertiary center. PoP had an incidence of 3.9% in all the abdominal surgeries in the study period. Surgical control of POP focus was achieved in 90.5% of cases and a complete abdominal clearance in 58.3%. Complication rate was 75% with a mortality of 26.2%. For MPI score, the ROC curve indicated a cut-off value of 29 with a sensitivity of 72.7% and specificity of 67.7% in predicting death. At univariate analysis, factors significantly related to poorer prognosis included advanced age (p 0.001), site of primary surgery (p 0.05), lack of abdominal clearance (p 0.003), generalized peritonitis (p 0.04) and high MPI score (p