AbstractWe aimed to elucidate details of comorbid chronic rhinosinusitis (CRS) in chronic eosinophilic pneumonia (CEP) under the collaboration between otolaryngologists and pulmonologists in a prospective study. The CEP diagnosis was performed by pulmonologists based on clinical symptoms, laboratory findings, and/or eosinophilia detected in bronchoalveolar lavage. All patients were referred to otolaryngologists before undergoing oral corticosteroid treatment for CEP. Ten CEP cases visited to otolaryngologists. All cases showed bilateral sinonasal inflammation in computed tomography (CT), indicating comorbid CRS. Nasal polyps (NPs) were observed in 50% of patients on endoscopy. Eighty percent of patients were diagnosed with eosinophilic CRS. In blood eosinophil levels and the mucosal eosinophil count, there were no significant differences between CRS without and with NPs. In Lund–Mackay CT total scores, among‐individual variability was observed in CRS with NPs. The collaboration revealed blood/sinonasal eosinophilia and the variability in Lund–Mackay CT total scores as remarkable findings about the comorbid CRS.