Arterial plasma amino acids were measured in 27 patients with serious septic complications after operation, 15 patients following reduction of femoral shaft fractures and 9 control patients on the 1st and 3rd days following uneventful major abdominal surgery. Amino acid concentrations in the controls were similar to those which were reported during early starvation. The amino acid patterns seen in all groups did not resemble that previously observed following glucocorticoid administration. In the patients with infection, mean phenylalanine concentration (108.0 .+-. 46.9 .mu.mol/l) was significantly greater than in the controls on the 1st (P < 0.001) or 3rd (P < 0.001) postoperative days. Of the septic patients with hyperphenylalaninemia, 4 also had elevated arterial methionine concentrations. These observations suggest that many of the patients with sepsis had seriously impaired liver metabolism. In patients with fractures, the concentrations of ornithine (P < 0.001), taurine (P < 0.05), and aspartic acid (P < 0.05) were lower than in controls. No other significant differences of amino acid concentrations were observed. It is difficult to relate these differences to a specific metabolic abnormality.