OBJECTIVE: We sought to determine whether histologic tissue invasion occurs in allergic fungal sinusitis (AFS) and, if so, to identify clinical indicators for the same. STUDY DESIGN AND SETTING: We conducted a retrospective case record review of all 28 AFS cases identified by histology over a 32-month period at a tertiary care referral center. All histologic specimens were reevaluated for features of invasive pathology, and case records were correlated for clinical, radiologic, or laboratory parameters associated with such invasion. RESULTS: In addition to the universal finding of the characteristic allergic mucin with fungal elements on histopathologic examination of the sinus luminal contents, 6 cases (21%) had additional evidence of mucosal invasion as indicated by granulomatous inflammation and branching septate fungal hyphae in the submucosal tissues. Such coexistent invasion was associated with advanced disease as indicated by a higher incidence of orbital involvement on clinical evaluation (P = 0.024), and extrasinus spread (intraorbital or intracranial spread) on the computed tomography evaluation (P = 0.003). The single death that occurred on follow-up was in a patient with coexistent invasion. CONCLUSION: Advanced AFS may be complicated by histologic evidence of tissue invasion. SIGNIFICANCE: The noninvasive and invasive forms of fungal sinusitis are not necessarily discrete and may coexist in the same patient. Clinical features of orbital involvement or computed tomography manifestations of extrasinus spread should alert the clinician to the possibility of invasion.