BACKGROUND: Chronic rhinosinusitis (CRS) is a common yet under-recognised chronic inflammatory disease of the nose and paranasal sinuses that is classified according to the presence (CRSwNP) or absence (CRSsNP) of nasal polyps. METHODS: This paper reports the methodology and descriptive results of the Global Allergy and Asthma European Network (GALEN) rhinosinusitis cohort. We established a large CRS cohort within the GALEN consortium (European FP6 research initiative) to identify inflammatory endotypes, the natural disease course, and its impact on health-related quality of life (HRQoL). Detailed information on the impact of CRS on HRQoL, comorbidity incidence, objective disease measures, and medical and surgical treatments were collected. RESULTS: This multicentre cross-sectional case-control study recruited 935 adults (869 eligible for analysis: 237 CRSsNP; 445 CRSwNP; 187 controls [reference group]). Comorbidities such as asthma, allergy, eczema, food allergy, urticaria, and chronic obstructive pulmonary disease were significantly more frequent in CRS patients. Nasal corticosteroids, antibiotics, and oral corticosteroids were the most common treatments. Significantly more CRSwNP patients reported previous sinonasal surgery. CONCLUSIONS: This study provides detailed information that facilitates studying CRS and its main phenotypes. However, patient distribution of this study does not necessarily reflect disease distribution in the general population.