Objectives: Rapid muscle wasting occurs during acute respiratory failure, resulting in muscle weakness and functional impairments. This study examines survivors' body composition in the year after acute respiratory distress syndrome and tests associations of patient characteristics, hospital exposures, and survivors' strength and physical functioning with whole body percent lean mass. Design: Prospective cohort study with 6- and 12-month follow-up. Setting: National study enrolling patients from five study centers. Patients: Acute respiratory distress syndrome survivors (n = 120). Interventions: None. Measurements and Main Results: Lean and fat mass from dual energy x-ray absorptiometry. On average, survivors gained whole body total mass (+1.4 kg; 0.1-2.7) and fat mass (+1.2 kg; 0.2-2.2) and maintained lean mass (+0.2 kg; -0.4 to 0.8) between 6 and 12 months. Proportionally, percent fat mass increased and percent lean mass decreased for the whole body, trunk, and legs (p < 0.05). Greater whole body percent lean mass was associated with younger age, male sex, and lower baseline body mass index, but not other patient characteristics or ICU/hospital exposures. Greater percent lean mass was also significantly associated with gait speed and 6-minute walk distance, but not volitional strength or self-reported functional status. Conclusions: In the first year after acute respiratory distress syndrome, patients gained fat mass and maintained lean mass. We found no association of whole body percent lean mass with commonly hypothesized hospital risk factors. Direct measurement of body composition and performance-based functional measures may be helpful for understanding functional recovery in ICU survivors.