Chronic rhinosinusitis (CRS) is a confusing disease for both allergists and otorhinolaryngologists, partly because of its poorly understood pathophysiology and partly because of its limited treatment options. Several recent reports have provided evidence for a better understanding of the etiology and the relationship of CRS to airborne fungi-especially to Alternaria. First, the development of novel methods enables detection of certain fungi in mucus from the nasal and paranasal sinus cavities. Second, a non-IgE-mediated immunological mechanism for reactivity of patients with CRS to certain common fungi has been described. Third, these fungi are surrounded by eosinophils in vivo, suggesting that they are targeted by eosinophils. Finally, the preliminary results of studies using antifungal agents to treat patients with CRS are promising. Overall, these recent discoveries provide a logical mechanism for the pathophysiology of CRS, and they also suggest promising avenues for treatment of CRS with antifungal agents.