The fatty acid composition of red blood cell (RBC) phospholipids in low-birth-weight infants was determined immediately after delivery and during the first 3 months of life. In the first study, infants were fed either human milk or two formulas with different fatty acid compositions but no long chain polyunsaturated fatty acids (LCPUFA). Both groups of formula-fed infants had significantly lower levels of docosahexaenoic acid (DHA) in RBC phospholipids compared with breast-fed infants. RBC phospholipid DHA was similar in the two formula groups at all ages. In the second study, infants received either a non-supplemented or a LCPUFA-supplemented formula. DHA remained stable in RBC phospholipids of infants supplemented with LCPUFA, whereas DHA decreased in RBC phospholipids of unsupplemented infants. These results confirm that adding DHA to formulas is more effective than increasing 18:3 n-3 content, in maintaining RBC phospholipid DHA levels.