Although Type I allergy is a trigger for provoking chronic inflammation, whether allergic sinusitis (AS) can be distinguished from sinusitis due to chronic infection is still debated. This study was performed to characterize inflammatory cells in AS and to determine whether patients with AS differ from patients with chronic suppurative sinusitis (CSS). 5 patients with AS and 10 patients with CSS were investigated. Cellular infiltration was studied using immunohistochemistry with monoclonal antibodies using CD3, CD4, CD8, CD25, major basic protein (BMK13), eosinophil cationic protein (EG2), neutrophil elastase, and tryptase. There were no differences in CD3+, CD4+, CD25+, and tryptase+ cells between the groups. Whereas the total number of eosinophils (BMK13+) also did not significantly differ, the number of activated eosinophils (EG2+) was significantly higher (P < 0.05) in patients with AS. Furthermore, a statistically significant increase in the percentage of activated eosinophils to total eosinophils (P < 0.05) was observed in patients with AS. In contrast, the number of neutrophil elastase+ cells was significantly higher (P < 0.05) in patients with CSS. These results suggest that patients with AS can be distinguished immunohistochemically from patients with CSS, with AS being distinguished by activated eosinophil infiltration.