The concentration in plasma of 15 fasting amino acids were measured in 14 control volunteers and 55 cancer patients. In addition, 16 patients (7 with, 9 without total parenteral nutrition [TPN] ) with metastatic sarcoma had sequential amino acid profiles measured during 6 weeks of ablative chemotherapy. In four cancer patient groups (lymphoma, sarcoma, osteosarcoma and metastatic sarcoma) with no or minimal weight loss, most plasma amino acid levels were similar to controls. Proline levels were significantly reduced in the lymphoma and sarcoma patients. Esophageal cancer patients with 20% body weight loss had a marked reduction in total and individual amino acid levels (except branched chain amino acids) compared to controls and all others. The metastatic sarcoma patients who received parenteral nutrition had higher levels of plasma lysine and tyrosine during chemotherapy than controls; however, TPN failed to change the majority of amino acid levels. It appears that plasma amino acid levels except proline were well maintained in cancer patients without weight loss. Esophageal cancer patients with weight loss demonstrated marked reduction in all circulating amino acids except branched chain. Parenteral nutrition did not significantly alter the amino acid profile of cancer patients undergoing chemotherapy.