Malnutrition is a condition characterized by a shortage or imbalance of energy and proteins and/or other nutrients. Malnutrition leads to measurable undesirable effects on body size and composition, functioning and clinical results. Criteria for malnutrition are eight loss, low BMI and loss of muscle mass. Patients with cancer should be repeatedly screened for malnutrition and its risk factors during the diagnostic and therapeutic treatment trajectory. Patients with cancer who are malnourished or are at risk of becoming so should be referred to a dietician for individualized dietary advice if they are undergoing curative treatment or before they do so. At the start of radiotherapy and/or chemotherapy treatment, tube feeding or parenteral feeding is given to all cancer patients who are malnourished or are at high risk of becoming malnourished. If the gastrointestinal tract is functioning well then tube feeding is preferable to parenteral feeding. Patients who are at an advanced stage of cancer and cancer patients who are anorexic or lose weight can be treated with megastrol acetate.