BACKGROUND Few cases of allergic fungal sinusitis have been systematically evaluated to conclusively confirm working clinical, histopathologic, and serologic diagnostic criteria. OBJECTIVES The objective of this study was to describe 67 consecutive cases of allergic fungal sinusitis, the largest number of cases yet published. METHODS Cases from 1 practice over 8 years were evaluated with a consistent protocol, including skin testing, serum chemistries and serologies, and surgical specimen analysis. RESULTS All patients were atopic (100 %) and had nasal polyposis (100%). They tended to be young (33.3+/-13.1 years, mean +/-SEM), immunocompetent (92 %; remaining 8 % with low quantitative immunoglobulin but normal function), have slight female preponderance (58%), have a history of hypertrophic rhinosinusitis (100%), report nasal cast production (75%), and have developed their disease in the southwestern United States. Bipolaris spicifera was the most prevalent fungus involved (67%). Total serum IgE (mean 668 IU/mL) and fungal-specific IgG were generally elevated, whereas fungal-specific precipitins and specific IgE were generally negative despite positive fungal-specific immediate hypersensitivity skin tests. CONCLUSIONS Patients with allergic fungal sinusitis tend to have elevated total serum IgE and fungal-specific IgG at diagnosis but not fungal-specific IgE or precipitins. Histopathologic criteria for allergic fungal sinusitis diagnosis are discussed. The southwestern United States appears to be a "hot spot" for the disease, particularly caused by B spicifera.