Although there is unanimity in the general public’s mind that childhood asthma is continuing to rise, evidence is accumulating that the prevalence has now plateaued, or may even be in gradual decline.¹ Whether this is real or confounded by changes in diagnostic labelling remains unclear. Whatever the prevalence, the underlying causes remain elusive, and suggestions are as numerous as research groups, each with their own agendas to promote. It is possible to build cases for a number of different environmental factors including varying exposures to specific allergens,² to bacterial and viral pathogens and their products (the hygiene hypothesis),^3,4 to environmental air pollution,⁵ to diet,⁶ and to obesity,⁷ among a range of ever increasing suggestions. An important issue for the public and for many health professionals, particularly those not familiar with the extensive primary literature, is to distinguish between those factors that might be relevant to causation and those that are responsible for provoking episodes of airflow obstruction in children with pre-existing disease. The reasons for this frequent failure to distinguish causation from exacerbation is understandable as factors known to exacerbate existing disease such as environmental tobacco smoke and exercise …