Background: Patients with advanced systolic heart failure are at risk of unintentional weight loss and muscle wasting. It has been observed that left ventricular assist device (LVAD) recipients gain weight after device implantation, although it is unknown whether this represents skeletal muscle mass gains. We aimed to determine whether skeletal muscle mass increases early during LVAD support. Methods: We prospectively recruited 30 adults with systolic heart failure ±21 days from LVAD implantation. Participants underwent whole-body dual X-ray absorptiometry to measure fat free mass, appendicular lean mass (ALM, lean mass in the arms and legs) and fat mass. Dual X-ray absorptiometry imaging was repeated at 3 and 6 months after LVAD implantation, with participation ending after the 6-month visit or heart transplantation, whichever occurred first. Changes in body composition were evaluated using mixed effects linear regression models. Results: The cohort was 87% male, with mean age 56±12 (SD) years, and mean body mass index 26.4±5.4 kg/m ² . Per sarcopenia ALM criteria, 52% of participants had muscle wasting at baseline. At baseline, mean fat free mass and ALM were 56.4±11.7 and 21.0±5.3 kg, respectively. Both measures increased significantly ( P <0.001) over 6 months of LVAD support: mean fat free mass change at 3 and 6 months: 2.3 kg (95% CI, 1.0–3.5) and 4.2 kg (95% CI, 2.2–6.1); mean ALM change at 3 and 6 months: 1.5 kg (95% CI, 0.7–2.3) and 2.3 kg (95% CI, 0.9–3.6). Conclusions: Among LVAD recipients with advanced systolic heart failure and high baseline prevalence of muscle wasting, there were significant gains in skeletal muscle mass, as represented by dual X-ray absorptiometry fat free mass and ALM, over the first 6 months of LVAD support.