Preoperative plasma aminograms constructed for 63 patients expected to undergo gastrectomy for gastric cancer at different stages showed markedly lower concentrations of many plasma amino acids in the Stage IV and recurrent cases. The amino acid levels were inversely proportional to tumor size. On the other hand, preoperative arteriovenous differences in free amino acid levels were positive in Stage I cancer but negative in Stage IV cancer, indicating that intake of amino acids by the skeletal muscles exceeded the outflow in Stage I, whereas there was a net loss of amino acids from the skeletal muscles in advanced cancer. The amount of amino acids actually lost from the skeletal muscles after muscular loading in Stage I cancer also surpassed that in Stage IV cancer. Administration of TPN solution supplemented with 31% branched-chain amino acids (BCAA) might favorably influence muscle protein metabolism in gastric cancer patients by inhibiting protein degradation and promoting synthesis, as treatment was more effective than 21% BCAA-enriched TPN.