Allergic fungal sinusitis is a benign noninvasive sinus disease related to a hypersensitivity reaction to fungal antigens. A wide variety of fungal agents has been implicated, with the vast majority belonging to the Dematiaceae family. Allergic fungal sinusitis should be suspected in any atopic patient with refractory nasal polyps. Sinus computed tomograms and magnetic resonance imaging findings can be quite distinctive, but not diagnostic. Diagnosis requires histopathologic examination, which shows characteristic allergic mucin. Hyphae can be demonstrated on special fungal stains or confirmed by a positive fungal culture. At surgery, the diagnosis should be considered if thick, tenacious allergic mucin is encountered in the atopic patient with nasal polyps. Fungal cultures should then be obtained, and the pathologist alerted to the possible diagnosis of allergic fungal sinusitis. Current recommendations for therapy include conservative but complete exenteration of all allergic mucin. This can often be accomplished endoscopically. Adjunctive short-term systemic steroids are often helpful, and nasal steroid sprays should be continued long term. The length and dose of steroid therapy is controversial. Persistence of allergic fungal sinusitis with recurrence of sinonasal symptoms is common, particularly when there has been incomplete eradication of allergic fungal mucin. Even when the patient is clinically disease free, recurrence can occur, presumably from reexposure to fungal antigens. Therefore close clinical, endoscopic, and radiographic follow-up is important.