The metabolic effects of an intravenous lipid emulsion containing medium chain triglycerides (MCTs) were studied in eleven critically ill, ventilated patients receiving parenteral nutrition. The effects were compared in a cross-over study with those of a conventional emulsion containing only long chain triglycerides (LCTs). The lipid was well tolerated, but there were differences in the metabolic effects with a significantly greater increase in the plasma concentrations of glycerol and ketones during MCT/LCT infusion compared to LCT. The plasma concentration of non esterified fatty acids was also higher. This fell rapidly post infusion. Since non esterified fatty acids and ketones are readily available energy sources for tissues lipid emulsions containing MCT may prove valuable for catabolic, critically ill patients.