A number of malignant tumors interact with the host to cause a syndrome of cachexia, characterized by extensive loss of adipose tissue and skeletal muscle mass, but with preservation of proteins in visceral tissues. Although anorexia is frequently present, the body composition changes in cancer cachexia cannot be explained by nutritional deprivation alone. Loss of skeletal muscle mass is a result of depression in protein synthesis and an increase in protein degradation. The main degradative pathway that has been found to have increased expression and activity in the skeletal muscle of cachectic patients is the ubiquitin-proteasome proteolytic pathway. Cachexia-inducing tumors produce catabolic factors such as proteolysis-inducing factor (PIF), a 24 kDa sulfated glycoprotein, which inhibit protein synthesis and stimulate degradation of intracellular proteins in skeletal muscle by inducing an increased expression of regulatory components of the ubiquitin-proteasome proteolytic pathway. While the oligosaccharide chains in PIF are required to initiate protein degradation the central polypeptide core may act as a growth and survival factor. Only cachexia-inducing tumors are capable of elaborating fully glycosylated PIF, and the selectivity of production possibly rests with the acquisition of the necessary glycosylating enzymes, rather than expressing the gene for the polypeptide core. Loss of adipose tissue is probably the result of an increase in catabolism rather than a defect in anabolism. A lipid mobilizing factor (LMF), identical with the plasma protein Zn-α2-glycoprotein (ZAG) is found in the urine of cachectic cancer patients and is produced by tumors causing a decrease in carcass lipid. LMF causes triglyceride hydrolysis in adipose tissue through a cyclic AMP-mediated process by interaction with a β3-adrenoreceptor. Thus, by producing circulating factors certain malignant tumors are able to interfere with host metabolism even without metastasis to that particular site. © 2004 Wiley-Liss, Inc.