The description of different plasma amino acid profiles for specific types of cancer suggests that the metabolic alterations brought about by each type of tumor determine their own, distinctive profile of plasma amino acids. However, the blood cell pool represents an important percentage of the total amount of amino acids and has been reported to undergo significant changes in several physiological situations, thus raising the question of what effect a situation like cancer could have on amino acid blood compartmentation. We determined the levels of individual amino acids in blood, plasma and blood cell compartment of 14 lung cancer patients, 16 breast cancer patients and the corresponding healthy controls (n = 14 and 18, respectively). Cancer, a situation of increased amino acid demand, was accompanied by a decrease in the amino acid availability, of which the blood cell pool would be the main contributor. Thus, the fact that the blood cell pool reflects more intensely than plasma the changes in amino acid availability and undergoes changes according to the demand of amino acids, reinforces the important role of the cell pool in blood amino acid compartmentation and handling. The profiles of blood amino acids characteristic of different types of tumors that have been proposed by some authors could be extended to other compartments-in addition to the plasma-and even be more informative.