Eighty per cent of all childhood asthma has its onset by 3 years of age. A similar percentage of childhood asthma is associated with atopy (presence of allergic rhinitis or eczema and/or positive skin prick tests). Defined risk factors for childhood asthma include allergen exposure, environmental tobacco smoke, viral respiratory illness, the presence of other atopic disease, and gender. Although a large percentage of children will have remission of asthma as they grow, they will retain the intermediate phenotypes of increased airway responsiveness and allergen and, therefore, be at risk of recrudescence of disease in adult life.