Breast-fed infants score better on visual and developmental tests than do formula-fed infants and this has been related to higher concentrations of erythrocyte docosahexaenoic acid (DHA, 22:6 omega 3). This prompted an investigation into the relationship between brain, retina, and erythrocyte fatty acids and diet in infancy. Total lipids of erythrocytes, retina, and brain cortex from 35 term infants were analyzed by capillary gas chromatography. Breast-fed infants had a greater proportion of DHA in their erythrocytes and brain cortex relative to those fed formula (P < 0.005) but differences were not observed in retina. Cortex DHA increased in breast-fed (but not formula-fed) infants with age (r2 = 0.72, P < 0.01, n = 15), largely an effect of length of feeding (r2 = 0.62, P < 0.01, n = 35). There was an association between age at death and erythrocyte DHA with cortex DHA (r2 = 0.50, P < 0.01). In contrast, accretion of cortex arachidonic acid was dependent on age but not diet. The higher concentration of DHA in brains of breast-fed infants may explain the improved neurodevelopment reported in breast-fed compared with formula-fed infants.