In order to evaluate the prognostic value of simple clinical parameters and the Mannheim Peritonitis-Index (MPI) 438 patients with abdominal infection entered a prospective study. 300 of them were managed by closed treatment with drainage, 138 by open treatment with planned relaparotomy. 63 patients (14.4%) died, 34 of them due to sepsis, 29 due to another cause. Stepwise logistic regression analysis showed only preoperative shock, concomitant disease and sepsis to be significant and independent factors leading to death. MPI had a strong correlation to mortality. Statistical validation showed a sensitivity of 88% and a specifity of 78% at the critical score of 26 points. The MPI is an easily documentated prognostic index for peritonitis with high accuracy in individual prognosis.