OBJECTIVE The pathogenesis of sinonasal polyps has not been known completely. We investigated the role of endothelial Nitric Oxide Synthase (eNOS) in the pathogenesis of sinonasal polyps. PATIENTS AND METHODS Study group (Groups 1-3) consisted of nasal polyp samples of patients with sinonasal polyps; and control group consisted of inferior turbinate samples of patients without nasal polyp. In Group 1: 14 specimens from ethmoid sinus; in Group 2: 10 specimens from nasal cavity; in Group 3: 10 specimens from maxillary sinus; and in Group 4 (Control): 9 specimens from inferior turbinate were included. By immunohistochemical staining technique, eNOS Positivity Index in mucosal layers; and in the inflammatory cells were assessed. RESULTS eNOS Positivity Index was higher at apical layer of epithelium; and perivascular and glandular parts of subepithelial layer. As a rate of mononuclear cells increased, eNOS positivity increased at basal part of epithelium. In eNOS Positivity Index of mononuclear cells increased ones, eNOS values also increased at glands of subepithelial layer. In nasal cavity, eNOS positivity index of all cells was significantly higher than that of the control group. Increased eNOS all cells positivity index values were seen with decreased glandular and endothelial eNOS values. In all cells group, fibroblasts were seen beside the mononuclear cells. It was observed that eNOS was not expressed in PMNC (mainly neutrophils), growing more in acute inflammatory process; and was expressed in MNCs and all cells group with fibroblasts which were the cells of chronic inflammatory process. Especially MNCs and fibroblasts may play a role in the polyp formation process. In males and in patients with longer polyp duration, eNOS values decreased. CONCLUSIONS We concluded that eNOS Positivity Index was higher at apical layer of epithelium; and perivascular and glandular parts of subepithelial layer. eNOS plays role in vascular dilatation, increases in vascular permeability; increases in nasal secretion due to glandular secretion; and edema in subepithelial and deep layers of the mucosa by affecting glands. Irritant agents in the breathing air and environment may cause increase in eNOS values at apical part of epithelium and may promote polyp formation by vasodilatation and increased glandular secretion due to increased nitric oxide values. In elderly patients and in long standing polyps, eNOS values decreased causing more fibrotic polyps.