Serum phospholipid eicosapentaenoic (PL-EPA) and docosahexaenoic acid (PL-DHA) concentrations are associated with the dietary intake of omega 3 fatty acids. PL-EPA and PL-DHA concentrations measured 4 y apart in 211 diabetic patients were highly correlated, with Spearman correlation coefficients of 0.49 (p = 0.0001) and 0.64 (p = 0.0001), respectively. PL-DHA was positively associated with Bayley psychomotor and mental developmental indexes (PDI and MDI, respectively) in preterm infants. Using multiple-regression analysis, 64% (R2 = 0.639; p = 0.0001) of PDI variance was explained by 1/DHA and weight at 1 y, whereas 82% (R2 = 0.816; p = 0.0001) of MDI variance was explained by weight at 1 y, Apgar score, 1/DHA, and 1/EPA. 1/DHA was negatively correlated with PDI and MDI, whereas 1/EPA was positively correlated with MDI. The results suggest that infant formulas should contain preformed DHA, and that a too-high supply of EPA in addition to DHA might be harmful in preterm infants.