In 24 critically ill patients HDL, LDL and VLDL values and those of apolipoproteins (apo) AI, B and E were evaluated just before (T0) and after 1 (T1), 3 (T3), 5 (T5) hours of intralipid infusion. Baseline levels of such parameters were compared with those obtained in 26 healthy volunteers. The critically ill patients showed lower values of apo AI and B (p less than 0.001) than control patients. In intensive care unit patients, a close relationship between HDL and apo AI (p less than 0.001) and between LDL and apo B (p less than 0.001) was observed in addition to the evidence of apo B and E interconversion. In fact, these apolipoproteins were always significantly correlated (p less than 0.01 a T0 e T3; p less than 0.001 a T1; p less than 0.05 a T5). The knowledge of the apolipoproteins functions could be important for the understanding and the detection of the negative effects that could follow to the infusion of fat emulsions.