It has not been established whether nutrition in early infancy affects subsequent neurodevelopment and function. If there is an effect, it seems probable that the essential fatty acids and their metabolites, the major constituents of brain structure, will be the most susceptible to dietary influence. We determined the phospholipid fatty-acid composition of cerebral cortex grey matter obtained from 20 term and 2 preterm infants who had died of "cot deaths" and related results to the milk diet the infants had received. Tissues were analysed by gas chromatography. The mean weight percentage of docosahexaenoic acid was significantly greater (p less than 0.02) in 5 breast-milk-fed infants (9.7%) than in 5 age-comparable formula-milk-fed infants (7.6%). In these formula-fed babies, the overall percentage of long-chain polyunsaturated fatty acids was maintained by increased incorporation of the major n-6 series fatty acids. In 1 formula-fed preterm infant, in whom the lowest concentration of cortical docosahexaenoic acid was found, the compensatory effect was only partial with both n-9 series eicosatrienoic acid or Mead acid and docosatrienoic acid also detected in the phospholipid. Supplementation of formula milks for term infants with docosahexaenoic acid and those for preterm infants with both docosahexaenoic and arachidonic acid could prove beneficial to subsequent neurodevelopment.