The concentration of 21 amino acids was measured in the venous plasma of 41 patients with cancer of the gastrointestinal tract who had lost weight, 12 patients who had lost a similar amount of weight from non-malignant, non-septic conditions (benign weight loss), 12 patients with cancer of the gastrointestinal tract who had not lost weight and 21 control patients. Sixteen patients with localised tumours were restudied eight and 12 weeks after successful removal of the primary growth. Six patients with pyloric stenosis (benign weight loss) were similarly studied for comparison after corrective surgery. The concentration of the amino acid arginine was significantly greater in patients with cancer both with weight loss (71.2 +/- 4.1 mumol/l mean +/- SEM) and without weight loss (66.8 +/- 4.1 mumol/l) when compared both with patients with benign weight loss (34.6 +/- 3.2 mumol/l) and with control patients (48.2 +/- 3.5 mumol/l) (p less than 0.05). In patients with cancer subjected to surgery the concentration of arginine (76.4 +/- 7.5 mumol/l) fell to normal levels eight weeks after operation and remained normal 12 weeks after surgery (48.6 +/- 4.4 mumol/l) (p less than 0.05). This was in contrast with the rise in plasma arginine in patients with pyloric stenosis after surgery, suggesting that the raised level of arginine was due to the presence of the primary tumour.