In recent decades it has become routine to describe asthma as an atopic disease. A theoretical paradigm has evolved in which allergen exposure produces atopic sensitisation and continued exposure leads to clinical asthma through the development of airways inflammation, bronchial hyperresponsiveness, and reversible airflow obstruction. As Martinez1 notes, this paradigm has been used with particular insistence with regard to house dust mite allergens, 2 3 but other allergens (cat, cockroach, dog) are also believed to be important. The importance of atopy is most widely accepted for asthma in children whereas, among adults, asthma has traditionally been divided into “extrinsic” and “intrinsic” asthma, although this also has been challenged. 4 It is acknowledged that not all cases of asthma fit this paradigm—for example, some occupational causes of asthma do not appear to involve atopy—but these are regarded as interesting minor anomalies that do not threaten the dominant paradigm.

In this review we assess the extent to which the development of asthma is attributable to atopy (we do not consider the separate issue of the extent to which the development of atopy itself is attributable to allergen exposure, although this is also the subject of debate1). We start by considering definitions of asthma and atopy and then review evidence on their association in random population surveys. We do not intend to argue that atopy does not play an important role in the development of a significant proportion of asthma cases. However, our concern is that the proportion of asthma cases attributable to atopy may have been overestimated, and that other possible aetiological mechanisms and risk factors for asthma may therefore have been neglected.