A view has recently emerged that the incidence of allergic diseases has gradually increased during the past century and now represents an important and growing health problem. The term atopy has been used to define a variety of allergic disorders including allergic rhinitis, asthma, eczema, and other less-well-defined allergic entities. More recently, the old definition of atopy based on IgE sensitization to common environmental allergens, as detected by in vitro immunoassay (eg, the radioallergosorbent test [RAST] or skin prick test), has provided a convenient tool for identification of the atopic state in epidemiologic studies. Most attention has focused on asthma, the allergic disease with the greatest clinical and economic burden, and this disease is attributed to atopy. The concept has emerged that aeroallergen exposure induces sensitization, followed by airway inflammation and clinical symptoms if exposure continues. The importance of atopy as the key factor for initiation and maintenance of clinical asthma is most widely accepted for childhood asthma, whereas adult asthma has been divided into atopic and nonatopic types. However, existing relationships among the features of clinical asthma, atopy, IgE, and the underlying immunopathology are not fully characterized and may not always be causal. This commentary discusses potential factors that may result in an immunopathology that is commonly attributed to atopy.