The understanding of how inhaled allergens affect the asthmatic patient has changed significantly over the years. Allergen inhalation was first recognized as a cause of asthma in 1873, and the clinically important late asthmatic response (LAR) was demonstrated, albeit crudely, in 1952. Immunopathogenesis of the LAR was controversial until the 1970s when it was demonstrated conclusively that allergen-induced late cutaneous response and LAR are immunoglobulin E dependent. Hargreave and colleagues were first to identify nonallergic airway hyper-responsiveness, a consistent feature of all types of asthma, as one important determinant of airway response to inhaled allergens. More important, airway responsiveness was shown to increase following an allergen exposure, primarily in association with the LAR. Inhaled allergens could now be recognized not as one of numerous stimuli triggering symptoms in asthmatics, but as a fundamentally more important stimulus that causes airway responsiveness and, as subsequent investigations showed, airway inflammation. This led to the important recognition that allergens are actually 'causes' of asthma.