Sepsis is characterized by an abnormal increase in plasma proline (PRO) level, which tends to be related to the severity of disease. This study has been performed to assess the relationship between changes in plasma PRO and levels and doses of other amino acids (AA) in critically ill septic patients undergoing total parenteral nutrition (TPN). Sixteen septic patients receiving TPN were randomly divided into two groups: 8 patients (Group A) received TPN with a standard AA solution, and 8 patients (Group B) with a modified AA solution (isonitrogenous, branchedchain AA enriched, with unchanged PRO concentration). Serial determinations of plasma AA profiles and of other variables were performed in each patient for a total of 396 measurements. In Group A mean plasma PRO level was 372μM/L; changes in PRO were tightly correlated with changes in the levels of most of the other AA, and the highest PRO levels characterized the more severely unbalanced septic metabolic profiles. In Group B, plasma levels of PRO and of the other AA (except glutamate, aspartate, taurine and the three branched-chain AA) decreased. The decrease in PRO level was well correlated with the increased branched-chain AA dose and with simultaneous decreases in plasma lactate and respiratory quotient. These changes could be related to a specific effect of branched-chain AA on septic metabolic derangement and on PRO metabolism, and to an improved balance between protein synthesis and catabolism.