SUMMARY The fatty acid composition of cholesterol (cholesteryl) ester, triglyceride, and lecithin has been investigated in whole plasma and individual lipoproteins of healthy control subjects, of patients with malabsorption, and also of patients without malabsorption. The results show a decreased proportion of the essential fatty acid linoleic acid in all three lipid classes in both groups of patients as compared with the healthy controls. This abnormality was more marked in the malabsorbers, especially those with steatorrhoea secondary to intestinal resection. Unequivocal biochemical evidence of essential fatty acid deficiency, as indicated by the appearance of 5, 8, 11 eicosatrienoic acid in plasma lecithin, was observed in two patients, both of whom had undergone major intestinal resections. The results suggest that intestinal resection predisposes to the development of essential fatty acid deficiency.The literature on the biochemistry and physiology of essential fatty acids and the syndrome of essential fatty acid deficiency in experimental animals is extensive and has been reviewed relatively recently (Alfin-Slater and Aftergood, 1968). Yet little is known about the occurrence of essential fatty acid deficiency in man, apart from the investigation carried out by Hansen, Wiese, Boelsche, Haggard, Adam, and Davis (1963) who described dermal changes, retardation of growth, and increased caloric requirements in previously healthy infants fed diets low in linoleic acid. Although decreased plasma essential fatty acid levels were reported in children with steatorrhoea due to cystic fibrosis of the pancreas (Kuo and Huang, 1965;Caren and Corbo, 1966) and coeliac disease (Rey, Fr6zal, Polonovski, and Lamy, 1965)