Progressive wasting is common in many types of cancer and is one of the most important factors leading to the early death of cancer patients. Although anorexia frequently accompanies cachexia it has been difficult to establish a simple cause-and-effect relationship, and nutritional supplementation is not able to effectively reverse the process of cachexia. An increased resting energy expenditure may contribute to weight loss in some cancer patients and may explain the increased oxidation of fat. Futile energy-consuming cycles, such as the Cori cycle, may contribute to the increased energy demand. Unlike starvation, weight loss in cancer arises equally from loss of muscle and fat, and the process is characterized by an increased catabolism of skeletal muscle and a decrease in protein synthesis. Several experimental studies have suggested a role for the cytokines tumor necrosis factor alpha, interleukins-1 and -6, and interferon gamma as mediators of the process of cachexia, although conclusive data supporting a role in human disease are often lacking. Catabolic factors capable of direct breakdown of muscle and adipose tissue appear to be secreted by cachexia-inducing human tumors and may play an active role in the process of tissue degeneration. Pharmacologic intervention using antagonists to cachexia factors may be capable of reversing the wasting process.