Objectives: This is the first study to report the presence of tryptase, a reflection of mast cell activity, in chronic middle ear effusion of patients whose atopic status was characterized. Design and Methods: Mediator activity of mast cells and eosinophils was measured prospectively from effusion of 33 randomly selected patients and 5 control subjects with chronic otitis media with effusion (OME). Atopy was determined by enzyme-linked immunosorbent assay. Middle ear biopsies from a second group of 8 OME patients and 4 controls were fixed in plastic and stained immunohistochemically for mast cells. Results: Sixty-one percent of patients had extensive activation of mast cells in their middle ears. Among those with elevated tryptase in their effusion, 95.6% were atopic and 94.7% also had elevated levels of effusion eosinophilic cationic protein (ECP). Tryptase levels were elevated only in the effusion of atopic patients, as compared with 5 controls (P < .01). Mast cells were present in 6 of 8 OME ears and absent in all 4 normal ears. Conclusion: Mast cells and its mediator tryptase, both indicators of a Th2-driven immune response, are present in a majority of ears that have chronic effusion. These findings support the hypothesis that middle ear mucosa is capable of an allergic response and that the inflammation within the middle ear of most OME patients is allergic in nature.