Preterm infants fed formula supplemented with DHA were shown in two randomized, clinical trials to have improved visual acuity in the first half of infancy. In the second clinical trial, infants simultaneously supplemented with DHA and provided with a nutrient-enriched preterm formula had a higher Bayley MDI score at 12 months than controls fed preterm formula. These data are the first evidence that DHA alone can also improve performance on early tests of mental development. Because visual and behavioural development are improved by providing this single dietary compound, DHA appears to be conditionally essential for preterm infants. Nevertheless, we would like to insert a few words of caution. More information about long chain n-3 and n-6 fatty acid requirements and balance for developing human infants is needed. As formulas are designed to meet n-3 and n-6 fatty acid needs, controlled studies of biochemistry and function should continue.