Seasonal allergic rhinitis (SAR) and asthma, which are frequently comorbid, share some common allergic pathogenic bases. Clinical manifestations of these disorders might therefore be viewed as local manifestations of a systemic inflammatory state. Not only do the onsets of allergic-rhinitis (AR) and asthma symptoms often coincide (within 1 year), but also nasal challenges with SAR allergens can induce airways hyperreactivity (AHR). Eosinophils, which are key effector cells in both SAR and asthma, cause AHR, tissue damage, and neuronal effects through secretion of toxic granule proteins, enzymes, and other mediators. The novel, nonsedating, histamine H1-receptor antagonist, desloratadine, which exerts various favorable effects on the allergic cascade, significantly decreased SAR symptoms (e.g., nasal congestion) and diminished daily beta2-agonist use and improved asthma symptoms, while maintaining pulmonary function, in patients with SAR-asthma who were treated with once-daily desloratadine regimens.