Chronic rhinosinusitis (CRS) is an inflammatory disorder affecting the nose and paranasal sinuses. Although bacteria have long been implicated as pathogens in most forms of CRS, it has been recognized that fungi may be responsible for some forms of CRS. Recent studies have shown that under optimal conditions, fungi can be identified within the nose and paranasal sinuses of nearly every individual. Considerable controversy exists concerning the proper diagnosis of and potential overlap between 'allergic fungal rhinosinusitis' and 'chronic rhinosinusitis'. Although the disease name 'allergic fungal rhinosinusitis' is suggestive of an immunoglobulin E (IgE) mediated reaction to fungi, recent studies demonstrate the presence of elevated serum IgE levels to one fungus while another fungus is present in CRS mucin of the same individual, questioning the role of type I hypersensitivity. Several mechanisms explaining the role of fungi in the pathogenesis of CRS, all requiring additional investigations with adequate controls, have been suggested and will be reviewed. Although preliminary trials suggest a beneficial effect of topical and oral antifungal agents in the treatment of CRS patients, several double-blind placebo controlled trials do not. Presently, in the absence of convincing immunological data and evidence of clinical improvement of CRS upon therapy with antifungal agents, the case against the fungus remains unproven.