Matrix metalloproteinases (MMPs) are known to play important roles in the invasion of nasal mucosa by inflammatory cells through degradation of extracellular matrix. Matrix metalloproteinase-9 (MMP-9) is considered to play a role in the pathogenesis of nasal polyp. The aim of the present study was to compare plasma MMP-9 levels of patients with nasal polyp of different etiologic origins, those with allergic nasal polyp (ANP) and non-allergic nasal polyp (NANP). In all, 29 patients (20 NANP and 9 ANP) and 20 healthy subjects were included in this study. Plasma MMP-9 levels were measured using ELISA. Plasma MMP-9 levels were higher (p<0.05) in ANP patients than in NANP patients and controls. In the group with ANP, plasma MMP-9 levels showed a positive correlation (p<0.05) with eosinophil counts. Plasma MMP-9 levels of NANP patients and control groups were similar (p>0.05). Moreover, that the highest levels of MMP-9 were in ANP patients may indicate the usefulness of this parameter in differentiating between the different etiologic origins of nasal polyp. Further studies are required to elaborate on the relationship between MMP-9 levels and allergic reactions. (C) 2008 Elsevier GmbH. All rights reserved.