Topical nasal corticosteroid therapy produces clinical improvement in patients with nasal polyposis. To examine the mechanisms of steroid effect, we quantified the number of inflammatory cell types as well as the expression of relevant cell markers in nasal polyps from steroid treated patients (n = 11) and polyps from untreated patients (n = 10) judged to require polypectomy for symptomatic relief. We found that the majority of eosinophils in these tissues were in the stromal layers, and that the proportion of activated eosinophils (EG2+ versus total eosinophil count) was significantly lower in polyps from patients treated with the topical nasal steroid budesonide, 200 to 400 micrograms/d for 1 mo, than in polyps from untreated patients. We also found that in polyps from treated patients, the superficial stromal layer (SSL) and deep stromal layer (DSL) both contained a significantly lower tissue density of lymphocytes of CD3, CD4, and CD8 subsets. Treatment was also associated with a lower, albeit nonsignificant, expression of ICAM-1 and HLA-DR in the epithelium. The tissue density of CD14+ cells (monocyte/macrophage) and of tryptase+ cells (mast cells) was the same in the two groups. These findings demonstrate tissue effects of topical corticosteroid treatment which may point to the mechanisms of therapeutic benefit. In addition, these observations illustrate the value of markers that indicate cell functional activity rather than just cell numbers.