The addition of branched chain amino acids (BCAA) to total parenteral nutrition (TPN) solutions of balanced aminoacid composition has been reported to result in improved nitrogen balance, preservation of plasma protein levels, and improved immune function; however, only a few large clinical studies have been performed, yielding conflicting results. We, therefore, studied in a prospective, randomized double blind trial, the effects of BCAA enrichment of an otherwise isonitrogenous and isocaloric TPN solution on plasma protein levels in 101 traumatized and septic patients. TPN-derived nitrogen intake was similar in both study groups. Nitrogen balances in the BCAA group (n = 49) were not different from those in the standard group (n = 52). Plasma total protein levels increased significantly in the standard group resulting in a significant difference between study groups on day 7. Pre-albumin levels increased significantly in the BCAA group and in the standard group. The number of neutrophils (rods) was significantly lower in the standard group compared to the BCAA group on day 7. Lymphocytes increased significantly in the standard group as did the number of platelets, resulting in a significant difference between study groups on day 7. We conclude that BCAA enrichment of standard TPN solutions does not result in more efficient nitrogen metabolism in septic or traumatized patients.