OBJECTIVE: To determine the predictive value of the index of peritonitis of Mannheim in patients with peritonitis in the Hospital Nacional Cayetano Heredia. PATIENTS AND METHODS: A prospective study appears, of 103 patients, greater of 14 years, with I diagnose of peritonitis, between November 2004 to April 2005. For its analysis I am used the square test of chi with coefficient of Pearson, and the test of T of student. For the analysis of the data two modalities were used, the first patients were divided in 3 groups, according to the value of the index of Mannheim, < 21, of 21 to 29, and > 29, and in 2 groups, d 26 and > 26 points. I am made considered of survival of Kaplan the Meier, using statistical program STATA 8.0 RESULTS: a mortality of 50% in patients with greater index of 26 points was obtained. One was a sensitivity 95.9%, a specificity of 80%, with positive a predictive value 98.9% and a negative predictive value of 50%. When considering 3 groups, < 21, 21-29 and > 29 points, was a mortality of 60% in patients with greater index of 29. I am made a survival curve obtaining itself a significant difference with a p=0, 0098. Figure 2. CONCLUSIONS: We found that the classification in 3 groups presents statistically significant difference, reason why recommended its use for the evaluation and beginning of aggressive measures.