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Background While inhalation therapy efficacy hinges on attaining proper peak inspiratory flow rate (PIFR), the prevalence of inappropriate PIFR among patients with chronic obstructive pulmonary disease (COPD) remains unstudied in Korea. This study aimed to assess the prevalence of inappropriate PIFR, its correlation with COPD assessment test (CAT) scores, and factors associated with suboptimal PIFR. Methods We enrolled 108 patients with COPD who had been using the same inhaler for at least 1 year without exacerbations. PIFR was measured using an inspiratory flow meter (In-Check DIAL G16). Demographic, clinical, pulmonary function, and CAT score data were collected. Inappropriate was defined as PIFR 90 L/min for aerosol device users. Results The cohort comprised 87 (80.6%) men, mean age 71.0±8.5 years, with mean post-bronchodilator forced expiratory volume in 1 second of 69.1%±1.8% predicted. Twenty-nine (26.9%) used aerosol devices only, 76 (70.4%) used DPIs only, and three (2.8%) used both. Inappropriate PIFRs were found in 17.2% of aerosol device users, and 42.1% of DPI users. CAT scores were significantly higher in the inappropriate PIFR group than in the appropriate PIFR group (11.2±7.7 vs. 7.5±4.9, p=0.003). In DPI users, female, shorter height, lower body weight and maximal voluntary ventilation (MVV) were associated with inappropriate PIFR. Conclusion The prevalence of inappropriate PIFR among patients with COPD is 17.2% for aerosol device users, and 42.1% for DPI users. Suboptimal PIFR correlates with female gender, shorter stature, lower weight and MVV in DPI users.