This thesis focusses on the relationship between allergy and soil-transmitted helminths (STHs). The term allergy is collectively used for atopic diseases and allergic sensitization in this thesis. Atopic diseases are inflammatory disorders characterized by hypersensitivity of the immune system to harmless environmental allergens. Atopic diseases include asthma, allergic rhinoconjunctivitis (hayfever), and atopic dermatitis (eczema). Allergic sensitization or atopy is the tendency to react to allergens and develop atopic diseases. Allergy is a worldwide problem, with asthma being the most common chronic disorder in children, and it poses a significant burden on the individual patient, family, healthcare services, and society. STHs are an important group of parasitic worms that often result in chronic infections in humans. The most important STHs are Ascaris lumbricoides (roundworm), Trichuris trichiura (whipworm) and hookworm (mine worm). More than two billion people worldwide are infected with these worms of which mostly children. STH infections cause malnutrition, anaemia, reduction in iron status, poor growth and delayed cognitive development. The prevalence of allergy has increased strongly in the last decades in affluent countries while it remains relatively uncommon in non-affluent countries. On the other hand, in these non-affluent countries STH infections remain an important problem while they are now uncommon in affluent countries. A causal relationship has been assumed between these opposite trends. This is part of the so-called hygiene hypothesis which assumes that experiencing childhood infections influences the immune system in such a way that these infections can reduce the tendency to develop allergy/ immune disorders. The relationship between STHs and allergy has been the focus of much epidemiological and immunological research. So far, research has provided conflicting evidence with studies showing that STH infections either promote, inhibit, or are unrelated to allergy. However, these results are mostly based on cross-sectional research. Therefore, the aim of this thesis is to assess within epidemiological studies the longitudinal effect of STH infections on allergy. The studies presented in this thesis have been performed in a cohort of Cuban schoolchildren, aged 4-14 at baseline, who were followed from 2003/2004 up to 2007. Allergy and STH infection status were evaluated every six to twelve months and STH infected children were treated with a single dose of 500 mg mebendazole, an anthelmintic drug. Atopic diseases were diagnosed using a validated questionnaire and allergic sensitization by skin prick testing. STH infection was assessed by detecting their eggs in stool.