Controversy continues over whether allergic fungal sinusitis represents a true allergy, an infection, or a point somewhere along a spectrum between allergy and infection. The present study describes two experiments that add weight to the argument that allergic fungal sinusitis (AFS) is truly an immunologically mediated hypersensitivity and not a form of infection. In the first experiment, eight patients with Bipolaris culture-positive AFS were prospectively evaluated with Bipolaris antigen skin testing and with inhibition radioallergosorbent (RAST) and enzyme-linked immunosorbent assay (ELISA) for Bipolaris-specific IgE and IgG antibodies. The Bipolaris AFS cases were compared with 10 control patients with no history of AFS. All eight AFS cases demonstrated positive skin testing to Bipolaris and in addition, all tested positive by RAST and ELISA for IgE and IgG Bipolaris antibodies, respectively. In the control group one patient had a positive skin test, ELISA, and RAST and one additional patient had a positive ELISA only. Good correlation was noted between skin test, RAST, and ELISA results. In the second experiment, sinus mucosa from 14 AFS patients and 10 control patients with other forms of surgical sinus disease was analyzed by immunohistocytochemistry for the eosinophilic inflammatory mediators major basic protein (MBP) and eosinophil derived neurotoxin (EDN) and the neutrophil mediator neutrophil elastase. All AFS cases demonstrated evidence of eosinophilic mediator release, and MBP and EDN predominated over neutrophil elastase. In the control group eosinophil and neutrophil mediator release in sinus mucosa was equal. The two experiments support the concept that AFS is an antigen-triggered, IgE- and IgG-mediated hypersensitivity response with a late-phase inflammatory reaction involving release of eosinophilic mediators.