Chronic rhinosinusitis is an inflammatory disorder with numerous predisposing factors, including genetics, pollution, anatomic anomalies, bacteria, and fungus. In some studies, fungi have been cultured from nearly 100% of patients with rhinosinusitis and nasal polyposis. T-cell sensitization to fungus with production of the T-helper 2 (Th2) cytokine interleukin-5 has been proposed as a key step in the initiation of the disease. Interleukin-5 also plays an important role in the effector phase of the disease through its effects on eosinophilopoesis and eosinophil survival. There is considerable controversy concerning the role of fungi as the cause of "chronic eosinophilic rhinosinusitis" and whether this even represents a distinct clinical entity. Further study is also required to define the optimal treatment for the disease.