Objectives: To determine the effect of increasing docosahexaenoic acid (DHA, 22 : 6 n-3) in breast milk on infant fatty acid pro®les. A secondary aim was to examine aspects of neural development. Design and Setting Double blind, placebo controlled study of infants recruited from postnatal wards at Flinders Medical Centre. Subjects: Fifty-two healthy term infants who were breast fed for at least 12 weeks and were from middle class families. Intervention: Breast milk with DHA concentrations that ranged from 0.1±1.7% of total fatty acids. This was achieved by supplementation of the maternal diet for the ®rst 12 weeks post partum. Results: Breast milk DHA was related to infant plasma (r 0.89, P`0.001) and erythrocyte (r 0.88, P`0.001) phospholipids in a saturable curvilinear manner so that breast milk DHA above 0.8% of total fatty acids resulted in little further increase in infant plasma or erythrocyte DHA levels. The rise in plasma and erythrocyte DHA was approximated by a fall in total n-6 polyunsaturated fatty acids. We could detect no relationship between visual evoked potential acuity (measured at 12 and 16 weeks) of infants by either the dietary grouping or the DHA status of individuals. A stepwise multiple regression showed that infant erythrocyte DHA at 12 weeks and home stimulation were the only independent factors associated with Bayley's MDI at 1 y (adjusted model r 2 0.18, P`0.005); while at 2 y gender and social score of the spouse were the only signi®cant predictors of Bayley's MDI (adjusted model r 2 0.22, P`0.005). Conclusions: Increasing breast milk DHA levels caused a dose dependent saturable increase in infant plasma and erythrocyte phospholipid DHA. There were no long-term effects of infant DHA status on indices of neurodevelopment.