The aim of this study was to evaluate whether the addition of immunological variables to the Physiological and Operative Severity Score for the enUmeration of Mortality and Morbidity (POSSUM) scoring system improves the predictability of postoperative mortality. One hundred and thirty-two consecutive patients who underwent moderate, major or major-plus elective surgical interventions for gastrointestinal tumors were scored using the POSSUM mortality risk analysis. Patients were placed in one of the two groups based on their POSSUM mortality rates which were either lower or higher than 5%. An additional 26 pre-operative and post-operative metabolic and immunological variables were measured and mortality-dependent variables were selected. Regression analysis with backward elimination of twelve pre-operative and post-operative variables correlating with POSSUM score revealed that postoperative neopterin, IL-6 and albumin were significantly dependent on the predicted mortality rates. According to these selected variables, the number of patients with a POSSUM predicted mortality rate higher than 5% increased from 64 to 88, but the percentage of the mean mortality decreased. Statistical differences between the original POSSUM and modified scoring system was highly significant (p<0.0001). The sensitivity and specificity of the modified scoring system was calculated to be 52.9% and 87.5%, respectively.