The last few decades have witnessed a considerable increase in atopic diseases, including asthma, allergic rhinitis and atopic dermatitis or eczema according to the new nomenclature, which collectively affect an estimated 20% of the world’s population – particularly children [1–3] . This increase in atopic disorders is not confined to industrialized countries but also affects developing nations. At first glance, atopic diseases appear to be a group of heterogeneous, unrelated disorders that share some risk factors. Indeed, the complex nature and etiology of these diseases have made it difficult to develop a unifying hypothesis to explain their steeply increasing prevalence worldwide. If two or more allergic diseases (asthma, eczema and allergic rhinitis) occur within one child, the question is whether this occurs purely by chance due to the high prevalence of these disorders or whether there are other factors at play. Furthermore, can the presence of atopy (defined as being positive for a specific IgE) contribute to the excess comorbidity frequently seen with these conditions? Pooled data from different European birth cohort studies including data from more than 10,000 children with information on underlying diseases and sensitization at 4 and 8 years of age showed that IgE sensitization is associated with the presence of excess comorbidity, irrespective of preexisting diseases  . However, the level of comorbidity is even higher in children with no sensitization. The authors concluded that the presence of sensitization accounted for <40% of comorbidity, suggesting that IgE sensitization can no longer be considered the dominating causal mechanism of allergic comorbidity.