SUMMARY Most infants in the UK are bottle fed with cows' milk formulae that are alleged to provide less than the minimum requirement for linoleic acid, i.e. 1 % of the total dietary energy. 20 term infants fed solely on a modified cows' milk formula that provided 0 * 55 % of the energy from linoleic acid were therefore examined for evidence of deficiency. Rates of growth in length and weight, measured during the first 3 months of life, were identical with those of 20 wholly breast-fed infants. Voluntary food intakes (kcal/kg per day) followed the normal pattern. The fatty acid composition of the plasma lipids showed changes characteristic of a low intake of linoleic acid, but triene:tetraene ratios did not indicate a deficiency state, and clinical signs of deficiency were not observed. These findings suggest that the requirement for linoleic acid is substantially less than was formerly believed. The small amount of a-linolenic acid present in cows' milk may however exert a sparing effect on linoleic acid.The experiments of Hansen and his colleagues Hansen et al., 1958;Wiese et al., 1958) clearly demonstrated that the human infant has a specific requirement for linoleic acid (18:2, n-6). Infants who were fed a skim-milk formula in which linoleic acid provided <0-1 % of the total energy developed a deficiency syndrome, the main features of which were (a) a reduction in the plasma concentration of linoleic acid and arachidonic acid (20:4, n-6) associated with an increase in the concentration of a triene; (b) impairment of energy use, shown by a pronounced rise in voluntary food consumption which was detected within 1-2 weeks; and (c) a characteristic dermatosis, a dry scaly thickened skin, which was apparent after 1-3 months of feeding. All symptoms disappeared promptly on treatment with the ethyl ester of linoleic acid, the so-called 'essential fatty acid'.In the body, linoleic acid undergoes alternating desaturation and chain elongation to produce its metabolically active derivative arachidonic acid. This process is not, however, confined to linoleic acid, but high concentrations of linoleic acid in the tissues appear to suppress the desaturation and chain elongation of other fatty acids. When the concen-