The relationship between polyunsaturated longchain fatty acids and atopy has been discussed for decades. Higher levels of the essential fatty acids linoleic acid and alpha-linolenic acid and lower levels of their longer metabolites in plasma phospholipids of atopic as compared to nonatopic individuals have been reported by several, but not all, studies. Largely similar findings have been reported in studies of cell membranes from immunological cells from atopics and non-atopics despite differences in methodology, study groups, and definitions of atopy. An imbalance in the metabolism of the n-6 fatty acids, particularly arachidonic acid and dihomo-gamma-linolenic acid, leading to an inappropriate synthesis of prostaglandin (PG) E2 and PGE1 was hypothesized early on but has not been corroborated. The fatty acid composition of human milk is dependent on the time of lactation not only during a breast meal but also the time of the day and the period of lactation. This explains the discrepancies in reported findings regarding the relationship between milk fatty acids and atopic disease in the mother. Prospective studies show disturbances in both the n-6 and n-3 fatty acid composition between milk from atopic and nonatopic mothers. Only the composition of long-chain polyunsaturated n-3 fatty acids was related to atopic development in the children, however. A relationship between lower levels of n-3 fatty acids, particularly eicosapentaenoic acid (20:5 n-3), and early development of atopic disease is hypothesized.