This review focuses on the studies investigating the prognostic implications of disease-related malnutrition. Malnutrition is a common problem in patients with chronic or severe diseases. Prevalence of hospital malnutrition ranges between 20% and 50% depending on the criteria used in order to determine malnutrition and the patient's characteristics. Furthermore, nutritional status is known to worsen during hospital stay which is partly due to the poor recognition by the medical staff and adverse clinical routines. Studies have repeatedly shown that clinical malnutrition however has serious implications for recovery from disease, trauma and surgery and is generally associated with increased morbidity and mortality both in acute and chronic diseases. Length of hospital stay is significantly longer in malnourished patients and higher treatment costs are reported in malnutrition. Since it has been demonstrated that proper nutritional care can reduce the prevalence of hospital malnutrition and costs, nutritional assessment is mandatory in order to recognise malnutrition early and initiate timely nutritional therapy.