Several studies have attempted to define nutritional parameters that can be used to select undernourished hospitalized patients and identify those who present higher risk of postoperative septic complications. Nevertheless, the majority of these studies do not take into consideration the severity of infectious episodes. Systems for scoring the severity of illness are of critical importance in hospital practice and clinical research. In fact, they could be used for the following purposes: (a) a more correct stratification of patients in clinical studies; (b) monitoring the evolution of the infectious complication; (c) prediction of survival; (d) optimization of the criteria for admission to and discharge from intensive care units; and (e) evaluation of quality, quantity, and costs of nutritional therapy in critically ill patients. In this article, the authors present a critical review on the clinical relevance of the methods of nutritional assessment more commonly used in clinical practice; and, moreover they discuss the more recently proposed systems for scoring the severity of illness.