The relationship between eosinophil and chronic sinusitis was investigated by measuring the concentration of eosinophil cationic protein (ECP). In addition to the blood, a cytological brush was inserted into middle meatus to scrape the nasal membrane, and the concentration of ECP in the collected specimen was measured. There was a moderate degree of correlation between the blood eosinophil count and the ECP concentration in the blood (r = 0.543, p < 0.01). On the other hand, there was high degree of correlation between the number of EG2-positive cells and ECP of the nasal membrane in the middle meatus (r = 0.805, p < 0.001), reflecting the amount of ECP in the nasal mucous membrane. The ECP in 10 control group subjects and 14 patients with severe chronic sinusitis and nasal polyps was significantly lower than that in 14 patients with severe chronic sinusitis and asthma (p < 0.01). The ECP concentration of the nasal membrane in the middle meatus was significantly lower in patients showing a good postoperative course compared with patients showing a poor postoperative course following Endoscopic Sinus Surgery (p < 0.01). These findings suggest that eosinophils are closely related to the pathology and clinical course of chronic sinusitis, and that the ECP concentration of the nasal membrane in the middle meatus reflects the pathology of chronic sinusitis. The ECP concentration thus has potential as a prognostic indicator for chronic sinusitis.