During the last years the most widely used lipid emulsion for parenteral nutrition has been based on soybean oil. This first generation of lipid emulsions used in TPN contained w-6 series polyunsaturated long-chain fatty acids (LCT) from soy, maize, sunflower and safflower oil. LCT contain an excess of linoleic acid which, when metabolised, produce large quantities of arachidonic acid and its metabolites. Although the generally used doses seem safe (1-2 g/kg/day by continuous perfusion), alterations in pulmonary function in patients with acute adult respiratory distress syndrome have been described, as have alterations in platelet function, hepatic function and haemodynamics, which are attributed to the excess of said metabolites. However, the most important side effect of the LCT lipid infusions is its influence on the immune response. Experimental and clinical studies show that LCT can interfere with various stages of the immune response such as the production of antibodies, complement synthesis, granulocytic and lymphocytic activity and the reticuloendothelial system. Various hypotheses have been formulated to explain the modulator effect of the polyunsaturated fatty acids on immune function: changes in the permeability of the cellular membrane, modifications in the synthesis of eicosanoids and the presence of peroxides derived from the oxidation of polyunsaturated fatty acids.