Objective assessment of symptoms in bronchiectasis is important for research and in clinical practice. The COPD Assessment Test (CAT) is a short, simple assessment tool widely used in COPD. The items included in the CAT are not specific to COPD and also reflect the dominant symptoms of bronchiectasis. We therefore performed a study to validate the CAT as an outcome measure in bronchiectasis. The CAT was administered to two cohorts of bronchiectasis patients along with other quality of life questionnaires. Patients underwent comprehensive clinical assessment. One cohort had repeated questionnaires collected before-and-after treatment of acute exacerbations. We analyzed convergent validity, repeatability, and responsiveness of the score and calculated the minimum clinically important difference (MCID) using a combination of distribution and anchor-based methods. In both cohorts there were positive correlations between the CAT and the St. George’s Respiratory Questionnaire (r = 0.90, P < .0001 and r = 0.87, P < .0001). There was an inverse relationship between CAT and Quality of Life – Bronchiectasis Respiratory Symptoms Scale (r = −0.75, P < .0001) and Leicester Cough Questionnaire score (r = −0.77, P < .0001). Patients with more severe disease, based on the bronchiectasis severity index, had significantly higher CAT scores. CAT also correlated with FEV1 % predicted and 6-min walk distance (6MWD). CAT increased significantly at exacerbation and fell at recovery. The intraclass correlation coefficient for two measurements four-weeks apart while clinically stable was 0.88 (95% CI, 0.73-0.95, P < .0001). An MCID of 4 was most consistent. CAT is a valid, responsive symptom assessment tool in bronchiectasis. The MCID is estimated as 4 points.