BACKGROUND: Weight loss and low lean body mass in patients with chronic obstructive pulmonary disease (COPD) are associated with increased mortality; however, the association between body mass index (BMI) and physical ability remains undetermined. We aimed to investigate the effect of BMI on activities of daily living (ADL) in hospitalized patients with COPD. METHODS: We used the Japan Medical Data Center's hospital-based database to extract data on hospitalized patients aged ≥ 20 years admitted for COPD between April 2014 and December 2018. The primary outcome was the presence or absence of deterioration in the Barthel Index score for ADLs at discharge compared with that at admission. Secondary outcomes were the length of hospital stay, readmission within 30 days, and the number of drugs administered on admission. RESULTS: We identified 6529 patients with COPD from the Diagnosis Procedure Combination database, of which 3476 were analyzed (excluded: n = 5, ages < 20 years; n = 3048, missing entries). Barthel Index scores tended to decline in patients with BMI < 18.5 kg/m2 (odds ratio, OR: 2.030, p < 0.001) and Hugh-Jones grade 4 (OR: 1.79, p = 0.05) and grade 5 (OR: 2.15, p = 0.01), but not in the obese group (p = 0.598). The low-weight group had a significantly longer hospital stay (p < 0.001) at 14 (8-25) days with greater readmissions within 30 days (6.3%, p < 0.001). CONCLUSION: Low BMI is associated with lower Barthel Index scores, longer hospital stays, requiring readmissions within 30 days of discharge.