Allergic Aspergillus sinusitis is a well-defined clinical and histologic entity, although surprisingly few reported cases have yielded any fungal growth on culture. Taking advantage of recent changes in the identification and classification of certain groups of fungi, we were able to identify a specific fungal organism in 19 of 22 consecutive patients with a histologic diagnosis of allergic fungal sinusitis over the past 2 1/2 years. Aspergillus was found in only one patient, while an organism in the family of dematiaceous fungi was found in 18 patients. Of these patients, the genus Bipolaris was the most commonly represented, while Exserohilum, Curvularia, and Alternaria species were seen with less frequency. Thus, it appears that Aspergillus may not be the most common etiologic agent in allergic "Aspergillus" sinusitis. Allergic fungal sinusitis is not unusual and its incidence may be increasing. On initial clinical evaluation it may be easily mistaken for malignancy or invasive fungal disease with the potential for overly aggressive treatment. Preoperative suspicion of allergic fungal sinusitis based on clinical and roentgenographic findings along with careful communication with the mycology laboratory about the possibility of dematiaceous fungal growth are necessary for proper diagnosis.