Malignant disease is often associated with weight loss and malnutrition. Nutritional support is frequently provided to patients with cancer in an attempt to improve nutritional status and reverse weight loss, with the aim of reducing morbidity and mortality rates. This review evaluates the effect of supplemental nutrition on morbidity and mortality in patients with malignancy undergoing treatment with surgery, chemotherapy or radiotherapy. It also assesses the effect nutritional supplementation has on host defence mechanisms and how nutrients affect tumour cell growth. The evidence suggests that perioperative nutritional support, if given for at least 10 days, reduces morbidity and mortality in patients with biochemical evidence of severe malnutrition, manifest as a low serum albumin concentration and excessive weight loss. In contrast, there is no evidence that parenteral nutritional support benefits patients undergoing chemotherapy or radiotherapy, in terms of either an increased tumour response rate or prolongation of survival. Current research on malignant disease is highlighting the role of specific nutrients (amino acids, essential fatty acids and polyribonucleotides) as key regulators of both anticancer host defence mechanisms and the control of nitrogen metabolism and tumour growth. Arginine, essential fatty acids and ribonucleotides have all been demonstrated to stimulate antitumour host defence mechanisms and some also modulate tumour cell metabolism. Dietary manipulation offers exciting possibilities for the innovative management of malignant disease.