Chronic rhinosinusitis (CRS), which is clinically classified into CRS without nasal polyps (CRSsNP) and CRS with nasal polyps (CRSwNP), shows considerable geographic differences and heterogeneity. Eosinophilic (E) CRS with nasal polyps (ECRSwNP) has a higher degree of disease severity and higher frequency of comorbid asthma. Epidemiologic studies in different ethnic populations have improved understanding of the pathophysiology of the disease. Here we report the clinical characteristics of Japanese patients with medically refractory CRS undergoing endoscopic sinus surgery (ESS). We recruited a total of 210 CRS patients and assessed them by nasal endoscopy, the Lund-Mackay score using computed tomography (CT), peripheral eosinophilia and smoking status. We also examined the comorbidity of asthma, effects of age and lung functions in the patients. In this study, 13% of CRSwNP patients and 20% of CRSwNP patients with peripheral blood eosinophilia exhibited obstructive lung dysfunction (FEV1/FVC <70%) despite the absence of an asthma diagnosis. Among elderly nonsmoker patients (≥ 60 years) who had never been diagnosed with asthma, 50% of CRSwNP patients with peripheral blood eosinophilia showed decreased FEV1/FVC <70%. Our findings suggest that asthma is under-diagnosed in CRS patients who undergo ESS, especially the elderly. Although the association between CRS and asthma has been recognized, increased attention to the comorbidity of obstructive airway diseases such as asthma is still needed for management of medically refractory CRS.