A large number of "biologic markers" for cancer have been described, including tumor-associated antigens, ectopic hormones, enzymes, and effects of tumor on the host's metabolism. Although tumors may metabolically differ from each other, they may induce similar derangements in glucose, lipid, and protein metabolism in the host. In particular, changes in carbohydrate metabolism may induce glucose intolerance that may be early and easily detected using an oral glucose tolerance test. Hypertriglyceridemia and reduced exogenous lipid clearance may represent an early marker of deranged lipid metabolism. Changes in protein metabolism, as reflected by plasma amino acid profile, may also represent a new diagnostic tool for cancer. Among other amino acids, free tryptophan seems to be the best candidate as a new useful marker for monitoring neoplastic disease. It is conceivable that, based on the understanding of the differences in plasma amino acid profiles, more specific and rational antineoplastic strategies may arise.