Rationale: Testing for underlying etiology is a key part of bronchiectasis management, but it is unclear whether the same extent of testing is required across the spectrum of disease severity. Objectives: The aim of the present study was to identify the etiology of bronchiectasis across European cohorts and according to different levels of disease severity. Methods: We conducted an analysis of seven databases of adult outpatients with bronchiectasis prospectively enrolled at the bronchiectasis clinics of university teaching hospitals in Monza, Italy; Dundee and Newcastle, United Kingdom; Leuven, Belgium; Barcelona, Spain; Athens, Greece; and Galway, Ireland. All the patients at every site underwent the same comprehensive diagnostic workup as suggested by the British Thoracic Society. Measurements and Main Results: Among the 1,258 patients enrolled, an etiology of bronchiectasis was determined in 60%, including postinfective (20%), chronic obstructive pulmonary disease related (15%), connective tissue disease related (10%), immunodeficiency related (5.8%), and asthma related (3.3%). An etiology leading to a change in patient's management was identified in 13% of the cases. No significant differences in the etiology of bronchiectasis were present across different levels of disease severity, with the exception of a higher prevalence of chronic obstructive pulmonary disease-related bronchiectasis (P