Nasal polyposis (NP) is a chronic inflammatory condition that is mostly characterized by an infiltration of eosinophils. How this eosinophilic inflammation leads to polyp formation remains largely unclear. In order to identify the most important factors in polyp growth, first we report the histologic features of two early stage manifestations of eosinophilic nasal polyps compared to their surrounding normal mucosa and mature polyps from the same patients. Histomorphologic analysis of these early stage manifestations of NP showed the presence of eosinophils, forming a subepithelial cap over a pseudocyst area that was filled with albumin. In mature NP, a large pseudocyst area containing albumin was surrounded by subepithelial eosinophilia. Second, in an approach to quantify and to study possible relations between eosinophilic inflammation and changes in extracellular tissue components we measured interleukin-5 (IL-5), eotaxin, eosinophil cationic protein (ECP), leukotrienes (LTC4/D4/E4), transforming growth factor-beta 1 (TGF-beta 1), fibronectin, hyaluronic acid, and albumin in nasal tissue homogenates of 31 subjects. Nasal polyp samples (n = 16) were obtained during routine endonasal sinus surgery, whereas control non-polyp samples (n = 15) from subjects with (6) and without (9) allergic rhinitis were obtained from the inferior turbinate during septum surgery. In the group of polyp patients 11 received no treatment, whereas 5 were treated with oral glucocorticoids (GCS) within 4 weeks before surgery. IL-5 was measurable in 8 of 11 untreated NP, whereas IL-5 could not be detected in all 15 controls nor in 4 of 5 oral corticoid-treated polyps. The comparison between the untreated polyp group and controls showed significantly higher concentrations of IL-5, eotaxin, ECP, and albumin in polyp supernatants, whereas TGF-beta 1 was significantly lower. In the oral GCS-treated group, ECP and albumin were significantly reduced compared to untreated nasal polyps. The same tendency, but not reaching significance, was seen for eotaxin and fibronectin, while no difference was found for LTC4/D4/E4 and hyaluronic acid between the groups. Our observations suggest a deposition of albumin (and possibly other plasma proteins) and extracellular matrix proteins, which may be regulated by the subepithelial eosinophilic inflammation, as a possible pathogenic principle of polyp formation and growth. IL-5 and eotaxin are found to be key factors for eosinophilic accumulation and activation in NP. Oral corticoid treatment may lead to the shrinkage of NP by downregulation of the eosinophilic inflammation and reduction of the extravasation and deposition of albumin in NP.