This review explores the effects of dietary long chain polyunsaturated fatty acids (LCPUFA) on various aspects of early immune development and their potential role in the development or the prevention of immune disease. Modern diets have become increasingly rich in n-6 LCPUFA and relatively n-3 LCPUFA deficient. These potentially "pro-inflammatory" dietary changes have clear implications for the immature and developing fetal immune system. It is now well known that immunological abnormalities precede the development of allergic disease and are frequently evident at birth or in the first months of life. This has lead to the hypothesis that potential effects of LCPUFA could be greatest in very early life before immune responses and clinical phenotype are established. Here we summarise the evidence that patterns of LCPUFA exposure in pregnancy can influence aspects of fetal immune in ways that are consistent with the immunological properties of these nutrients in adults. Specifically, human studies have shown that higher levels of n-3 LCPUFA are associated with reduction in neonatal oxidative stress, reduced production of inflammatory leukotienne B4 (LTB4) and altered T cell function. Inverse correlations between n-3 LCPUFA levels and neonatal T cell cytokine production, are consistent with adult studies showing reduction in T cell cytokine production with fish oil supplementation. At this stage the relevance of these effects in the prevention of disease is unclear. Although there have been no effects of postnatal fish oil supplementation (from 6 months of age) on allergy prevention, preliminary studies suggest possible merits in pregnancy and there are ongoing pregnancy intervention studies to address this more definitively.