In malnutrition lipoprotein lipase activity in the liver and the beta-lipoprotein concentration in serum are reduced. The lipolysis is increased. One suspected a reduction of fat elimination in such patients. In 10 malnourished patients with a mean body weight of 73% ideal body weight an intravenous fat tolerance test was performed. The elimination rate was in mean 5.77%/min, that is in the normal range. Studies with radioactive triglycerides showed a metabolization of 30% of infused fat during 24 hours. These results were confirmed by measurements of gas exchange. Energy expenditure and nitrogen balance in malnourished patients correlate with the energy intake but not with the amount of fat in the parenteral nutrition. That could be confirmed in a prospective randomized study in 8 malnourished patients. The carbohydrate oxydation provided 70% to 80% of total energy expenditure independent from the dosage of fat in these patients. The minimum of fat needed in total parenteral nutrition to avoid essential fatty acid deficiency is 100 g per week. The tolerable maximum dose ist not known. In 10 malnourished patients with a mean time of 36 days of parenteral nutrition a dosage of 2.5 g/kg X d-1 was well tolerated and no side effects were seen. In 3 of these patients a previous cholostasis improved during parenteral nutrition.