To determine the effect of maternal dietary n-6 and n-3 polyunsaturated fatty acids (PUFA) on the amounts of these fatty acids in human milk, two criteria must be met. These are assessment of the maternal diet and accurate analysis of the milk fatty acids. This type of analysis requires gas-liquid chromatography with capillary columns to resolve important n-6 and n-3 C20 and C22 fatty acid. This type of analytic equipment has only recently become available; thus the amount of complete data on human milk fatty acids is limited. To assess actual fatty acid intakes by the infant, the fat content and volume of milk received by the infant must be known. Alterations in maternal dietary intake of PUFA cause similar changes in milk PUFA. Several investigators have shown that maternal supplementation with fish oils increases the amounts of 20:5n-3 and 22:6n-3 in milk and maternal and infant erythrocyte lipids. A new mathematic index for assessment of essential fatty acid status, the mean melting point of plasma phospholipid fatty acids, has been proposed. We found in some mother-infant pairs that maternal supplementation with fish oil lowered the mean melting points of erythrocyte lipids to levels seen in nonpregnant women.