Background: Nasal polyps (NPs) are characterized by eosinophilic inflammation, which is generally considered sensitive to corticosteroid treatment. Objectives: We evaluated levels of neutrophilia in NPs and investigated whether increased neutrophilia in polyp tissue affected the response to corticosteroid treatment. Methods: We studied 3 independent cross-sectional groups of patients with NPs. Levels of infiltration by different types of inflammatory cells were determined by using immunohistochemical analyses and compared with those seen in control nasal tissues from subjects without NPs. Levels of inflammatory mediators were measured by using real-time PCR, ELISA, and FlowCytomix analyses. Patients with NPs received oral corticosteroid therapy (30 mg of prednisone once daily for 7 days); clinical parameters of efficacy were associated with NP phenotypes. Results: Among patients with NPs, 76.5% had an eosinophilic phenotype, 46.0% had a neutrophilic phenotype, and 35.8% had a mixed phenotype (indicated by double staining). Overall, patients' symptoms improved after corticosteroid treatment; numbers of eosinophils and levels of their mediators (IL-4 and IL-5), but not numbers of neutrophils or levels of their mediators (IL-8 and interferon-inducible protein 10), were reduced (P < .05). After corticosteroid treatment, patients with the nonneutrophilic phenotype (neutrophil negative) had significantly greater reductions in bilateral polyp size scores, nasal congestion scores, total nasal symptom scores, and nasal resistance than patients with the neutrophilic phenotype (neutrophil positive, P < .05). Conclusions: There are different phenotypes of NPs based on the type of immune cell infiltrate and cytokines produced (eosinophilic or neutrophilic). Patients with the neutrophilic phenotype have less response to treatment with corticosteroids based on symptom scores. (J Allergy Clin Immunol 2012;129:1522-8.)