In a prospective controlled clinical study 30 patients with moderate degree of malnutrition, normal liver and kidneys, and a functioning gastrointestinal tract were randomized to receive a free amino acid and small peptide enteral diet (15 patients) or an isonitrogenous isocaloric parenteral support for at least 10 days (total energy: 2900 kcal, nitrogen: 14.5 g, carbohydrates: 380 g, fat: 112 g, N/non protein calories: 1/175). The parenteral and enteral diets had the same protein/lipid/carbohydrate composition. The data indicated that both routes led to positive nitrogen balance. Nitrogen equilibrium was achieved by day 3 in the TPN group and by day 5 in the enteral group. There were no significant changes in serum albumin within either group. Serum level of transferrin reached a significant increase in both groups (p = 0.003). Thyroxine-binding prealbumin rose significantly in both groups as well (p = 0.019 and 0.004 respectively). Statistically significant rises in lymphocyte counts (p = 0.003 and 0.001 respectively), in levels of C3 (p = 0.009 and 0.001 respectively), IgA (p = 0.002), IgG (p = 0.004 and 0.003 respectively) and IgM (p = 0.004) occurred in either treatment group. There was a high incidence of negative skin tests at the start of the study in the enteral group (73.3%) and the TPN group (60%). By the end of the study the incidence of negative results for this test was 40.0% and 26.6% respectively. Despite maintenance of similar glucose levels in both groups, TPN led to significantly (p = 0.000) higher serum insulin levels. The serum insulin increased almost linearly over the study period, and eventually prevented fat mobilization and lipolysis, so that free fatty acid levels had fallen significantly (p = 0.000). A significant elevation of the liver enzymes over the study period occurred in the TPN group, but not in the enterally fed patients. The present findings provide no evidence that semi-elemental diets are in any way inferior to isonitrogenous isocaloric regimes parenterally given for a short period of time.