The Melbourne Atopy Cohort Study (MACS) is a longitudinal study of a high-risk birth cohort, and their families, that focuses on the natural history, causes and consequences of allergic diseases (eczema, food allergy, asthma and allergic rhinitis). There is evidence that the prevalences of asthma, eczema and allergic rhinitis have risen rapidly since the 1960s and, although the evidence is less robust, there also appears to have been a later increase in the prevalence of food allergy. Australia has one of the highest rates of these diseases in the world. The rapid rise in prevalence suggests that shifts in environmental exposures are important in the induction of these diseases, as genetic influences alone cannot account for such swift changes.