INTRODUCTION: Patients with chronic airway disease may present features of both asthma and COPD, commonly referred to as asthma-COPD overlap syndrome (ACOS). Recommendations on their diagnosis are diffuse and inconsistent. This survey aimed to identify consensus on criteria for diagnosing ACOS. METHODS: A Belgian expert panel developed a survey on ACOS diagnosis, which was completed by 87 pulmonologists. Answers chosen by ≥70% of survey respondents were considered as useful criteria for ACOS diagnosis. The two most frequently selected answers were considered as major criteria, others as minor criteria. The expert panel proposed a minimal requirement of two major criteria and one minor criterion for ACOS diagnosis. Respondents were also asked which criteria are important for considering inhaled corticosteroids prescription in a COPD patient. RESULTS: To diagnose ACOS in COPD patients, major criteria were "high degree of variability in airway obstruction over time (change in forced expiratory volume in 1 second ≥400 mL)" and "high degree of response to bronchodilators (>200 mL and ≥12% predicted above baseline)". Minor criteria were "personal/family history of atopy and/or IgE sensitivity to ≥1 airborne allergen", "elevated blood/sputum eosinophil levels and/or increased fractional exhaled nitric oxide", "diagnosis of asthma <40 years of age"; "symptom variability", and "age (in favor of asthma)". To diagnose ACOS in asthma patients, major criteria were "persistence of airflow obstruction over time (forced expiratory volume in 1 second/forced vital capacity ratio <0.7)" and "exposure to noxious particles/gases, with ≥10 pack-years for (ex-)smokers"; minor criteria were "lack of response on acute bronchodilator test"; "reduced diffusion capacity"; "limited variability in airway obstruction"; "age >40 years"; "emphysema on chest computed tomography scan". CONCLUSION: Specific criteria were identified that may guide physicians to a more uniform diagnostic approach for ACOS in COPD or asthma patients. ...