Allergic fungal rhinosinusitis is a phenotype of chronic rhinosinusitis with nasal polyposis, characterized by type 1 hypersensitivity to fungi, eosinophilic mucin with fungal hyphae in sinus secretions, and propensity for mucocele formation and bone erosion. Although its differentiation from other forms of chronic polypoid rhinosinusitis with eosinophilic mucin is sometimes problematic, type 1 hypersensitivity is a component of the disease process. Medical and surgical management can be augmented by immunotherapy directed toward the patient's specific allergen sensitivities. The primary rationale for immunotherapy is to control the allergic diathesis that may be contributing to the patient's chronic sinus inflammation.