Introduction. The atopic diseases of asthma, eczema and hayfever are immunologically mediated diseases initiated and perpetuated by T-helper (Th) cells of the Th2 phenotype, in affluent countries the prevalence of atopic diseases has increased dramatically in the last 40 years. This increase has been attributed to declining dietary intake of naturally occurring antioxidants. Epidemiological data and in vitro studies demonstrating that antioxidant deficiency is associated with Th2 differentiation support this 'antioxidant' hypothesis. The 'hygiene' hypothesis attributes the recent increase in atopic disease to declining childhood infections. This proposal originated from the observation of an inverse association between atopic disease and birth order. There is increasing interest that in utero influences play a critical role in determining the development of childhood atopic disease. There are reports that Th-cells from genetically susceptible neonates and neonates who subsequently develop atopic disease exhibit altered in vitro proliferative and cytokine responses. A proposed in utero influence is the sensitisation of fetal Th- cells by allergens and this concept is supported by indirect evidence. This thesis describes a prospective study designed to test the hypothesis that maternal allergen exposure and dietary antioxidant intake during pregnancy influence fetal Th-cell differentiation and the subsequent development of childhood atopic disease. Methods. Advantage was taken of a prospective cohort study of 2,000 pregnant women. Cord blood samples were obtained from a random sample of the neonates at birth. Cord blood mononuclear cells (CBMC) were stimulated with mitogen, control antigens and allergens. CBMC proliferative responses were quantified and CBMC interleukin-4 (IL-4) and interferon-y (IFN-y) secretion was measured by the celELISA method. These responses were related to antenatal data collected prospectively relating to the pregnancy. In a small number of samples the CD45 isoform of the responding CBMC was determined to investigate whether in utero allergen sensitisation of fetal Th-cells occurs. CBMC responses were also related to respiratory symptoms and atopic disease during the first year of infant life. Results. CBMC responses from 223 cord blood samples were characterised. CBMC culture conditions were optimised and the celELISA successfully detected secreted IFN-y and IL-4. CBMC proliferative responses were detected in 27-91% of cord blood samples, IL-4 responses in 18-31% and IFN-y responses in 19-88%. CD45 isoform analysis indicated that in utero sensitisation of timothy grass specific Th-cells occurs in 38% of pregnancies. CBMC proliferative responses were positively associated with a family history of atopic disease and maternal smoking. CBMC proliferative responses were negatively associated with birth order and maternal dietary vitamin E intake during pregnancy. CBMC IFN-gamma and IL-4 cytokine responses were positively associated with each other and a family history of atopic disease. CBMC IFN-gamma responses were negatively associated with birth order. Wheezing illness in the first year tended to be associated with increased CBMC proliferative responses at birth. Conclusions. Previously identified risk factors for atopic disease, which have been considered to be manifestations of post-natal influences, exert significant antenatal influences. The accepted adverse effects of maternal smoking on children's respiratory health may be a consequence of in utero influences. This study demonstrates a major in utero component to the association between birth order and atopy, contradicting the widely held assumption that it is a consequence of the protective effect of childhood infections. The maternal influence of dietary vitamin E intake raises the possibility of preventing childhood allergy by modifying the maternal diet during pregnancy. This study also provides the most direct evidence to date of the in utero sensitisation of Th-cells by allergens.