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Fungi are important etiologic agents of sinusitis. However, features of fungal sinusitis including the histologic spectrum, diagnostic mishaps, incidence, and fungal types have not been systematically studied. From 1996 through 2001, a total of 788 surgical pathology sinus specimens from 384 cases was retrieved. Fungal sinusitis was diagnosed in 58 specimens (7%) from 47 cases (12%). Four histologic categories of fungal sinusitis were identified: (1) allergic fungal sinusitis in 34 cases (copious mucin, abundant eosinophils, Charcot-Leyden crystals (so-called allergic mucin), with rare noninvasive fungal hyphae); (2) mycetoma/fungus ball in 11 cases (tightly packed fungal hyphae without allergic mucin or tissue invasion); (3) chronic invasive fungal sinusitis in 1 case (tissue granulomas with fungal hyphae); and (4) acute fulminant fungal sinusitis in 1 case (fungal vascular invasion). The diagnosis was initially missed in 16/34 (47%) cases of allergic fungal sinusitis despite typical features; incorrect classification was noted in 47% of cases. Sixty-seven percent of cases had positive fungal cultures, dematiaceous fungi being the most common. Allergic fungal sinusitis accounted for the majority of fungal sinusitis. Although misdiagnosis or incorrect classification is rather frequent for fungal sinusitis, awareness of the distinctive morphologic features of this entity may prevent these errors.