BACKGROUND: The impact of bronchiectasis severity on the outcomes of patients with allergic bronchopulmonary aspergillosis (ABPA) remains uncertain. OBJECTIVE: To evaluate whether bronchiectasis severity is associated with an increased risk of ABPA exacerbations. METHODS: We retrospectively analyzed ABPA patients between 2007 and 2019. Patients were categorized based on the segments involved by bronchiectasis as mild (1-5), moderate (6-9), and extensive (≥10). We compared lung function and immunological markers among the groups. A multivariable Poisson regression analysis, using follow-up duration as an offset variable, assessed the association between bronchiectasis severity and ABPA exacerbations, adjusting for key confounders. We report the association as an adjusted relative rate (aRR) with 95% confidence intervals (CI). RESULTS: We included 705 ABPA patients (mean age, 35 years). Of these, 219 (31.1%), 226 (32.1%), and 260 (36.9%) had mild, moderate, and extensive bronchiectasis. Patients with extensive bronchiectasis had poorer lung function and elevated immunological markers (serum total IgE, A.fumigatus-IgE, and -IgG) than those with mild or moderate bronchiectasis. The exacerbation frequency increased with the severity of bronchiectasis (mild: 41.5% vs. moderate: 53.4% vs. extensive: 57.7%, p=0.005). On multivariable analysis, the risk of ABPA exacerbation increased significantly with bronchiectasis severity (aRR [95% CI]
extensive:1.51 [1.09-2.08], moderate: 1.50 [1.09-2.08]). Additionally, increasing age (aRR, 0.84 [95% CI, 0.76-0.94]) and BMI (aRR, 0.97 [95% CI, 0.94-0.99]) were associated with lower exacerbation risk after adjusting for total IgE, lung function, and high-attenuation mucus. CONCLUSION: Moderate-to-extensive bronchiectasis is associated with worse lung function, heightened immunological severity, and an increased risk of ABPA exacerbation.