Objectives To investigate the impact of body mass index on activities of daily living in inpatients with acute heart failure. Design A retrospective cohort study. Setting A hospital-based database contains Diagnosis Procedure Combination survey data from 100 participating acute-care hospitals. Participants 11,301 inpatients aged 20 year or older who were admitted to the participating hospitals with a diagnosis of acute heart failure. Measurements The Barthel Index score at discharge and hospital death. Results The number of patients with a body mass index of <18.5 kg/m2 (underweight), 18.5–22.9 kg/m 2 (low–normal weight), 23.0–24.9 kg/m 2 (high–normal weight), 25.0–29.9 kg/m 2 (overweight), and ≥30.0 kg/m 2 (obesity) were 1689 (15%), 4715 (42%), 1809 (16%), 2306 (20%), and 782 (7%), respectively. Median Barthel Index scores at admission and discharge were 65 and 100, respectively. Hospital death occurred in 101 (0.9%) patients. Lower body mass index was associated with lower Barthel Index score at discharge and higher mortality. Multivariable analysis adjusted for body mass index, age, sex, New York Heart Association classification, Barthel Index score at admission, the updated Charlson Comorbidity Index, length of hospital stay, number of drugs administered, and rehabilitation during hospitalization revealed that body mass index was independently associated with Barthel Index score at discharge (beta: 0.354; 95% confidence interval: 0.248–0.461) and hospital death (odds ratio: 0.926, 95% confidence interval: 0.877–0.978). Conclusion Overweight and obese inpatients showed greater independence in activities of daily living at discharge and lower rates of mortality, indicating the obesity paradox. A combination of rehabilitation and improved nutrition seems to be important in underweight patients with acute heart failure.