There may be a causal relationship between intake of n-6 polyunsaturated fatty acids (PUFAs) and childhood allergic diseases. This can be explained by plausible biological mechanisms involving eicosanoid mediators produced from the n-6 PUFA arachidonic acid. Long chain n-3 PUFAs are found in fish and fish oils. These fatty acids act to oppose the actions of n-6 PUFAs. Thus, it is considered that n-3 PUFAs will lower the risk of developing allergic diseases. In support of this, protective associations have been reported between maternal fish intake during pregnancy and allergic outcomes in infants and children from those pregnancies. However, studies of fish intake during infancy and childhood and allergic outcomes in those infants or children are inconsistent, although some reported a protective association. Supplementing pregnant women with fish oil can induce immunologic changes in cord blood. This supplementation has been reported in some studies to decrease sensitisation to common food allergens and to lower the prevalence and severity of atopic dermatitis in the first year of life. The protective effect of maternal n-3 PUFAs may last until adolescence of the offspring. Fish oil supplementation in infancy may decrease the risk of developing some manifestations of allergic disease, although this benefit may not persist. Whether fish oil is a useful therapy in children with asthma receiving standard therapy is not clear from studies performed to date and this requires further exploration.