Conclusion. These findings suggest that patients with both sinusitis and asthma present the histopathologic characteristic of a marked chronic inflammatory reaction, and that eosinophil infiltration may play a significant role in this marked inflammation of the sinus mucosa. Objective. Chronic sinusitis and bronchial asthma are known to be closely related. However, the appearance of the mucosa in chronic sinusitis patients with asthma is somewhat different from that in patients without asthma. Material and methods. We compared the sinus mucosal histopathologies of asthmatic patients with those of non-asthmatic patients. Fifty-three sinusitis patients with a diagnosis of asthma and 54 sinusitis patients without asthma, who served as controls, were enrolled in the study. All of these patients underwent endoscopic sinus surgery. The following seven light microscopic findings were compared in the asthmatic and non-asthmatic groups: the thickness of the basement membrane, goblet cell hyperplasia, subepithelial edema, submucous gland formation, eosinophil infiltration, lymphocyte infiltration and polymorphonuclear leukocyte infiltration. In addition, we explored a possible link between asthma and sinusitis by comparing the following factors in asthmatic and non-asthmatic patients: the presence of allergy, the degree of preoperative polyposis and the extent of preoperative disease as scored by means of ostiomeatal unit CT findings. Results. No statistically significant differences were found between the two groups in terms of the presence of allergy, the degree of preoperative polyposis or the extent of preoperative disease. Basement membrane thickening, goblet cell hyperplasia and eosinophil infiltration were more prominent in the asthmatic compared to the non-asthmatic group (p <0.05). © 2005 Taylor & Francis.