Nasal Polyps (NPs) are the most common mass lesions found in the nose. NPs cause airway obstruction, prevent normal sinus function, and can lead to infection of the eye, facial bones and central nervous system. The predominant cell type inhabiting NPs is the eosinophil, and the chemokine eotaxin is believed to play an important role in NP eosinophilia. The objective of this study was to localize and quantitate expression of eotaxin mRNA in human NPs. Total RNA was isolated from NPs that were collected from 5 patients who had undergone polypectomy. Portions of these polyps were also fixed in formalin, embedded in paraffin, and sectioned onto slides for use in in situ hybridization. Total RNA from one patient was used in a reverse transcriptase polymerase chain reaction using eotaxin specific primers to generate a human eotaxin cDNA. The eotaxin cDNA was cloned and used to generate probes for Northern blot analyses and for use in in situ hybridization (ISH). Eotaxin mRNA was detected by Northern analyses in all patient samples, though the relative expression level in each patient varied. ISH localized the expression of eotaxin mRNA specifically in eosinophils in 2 of the 3 patients in the study for whom the embedded polyp tissue appeared sufficiently well preserved for mRNA localization. Our findings suggest that eosinophilia in NPs is likely a self-amplification process whereby increasing numbers of eosinophils are recruited to enter the polyp as a result of production of eotaxin by eosinophils already within the polyp.