Introduction: Patients with advanced systolic heart failure (HF) 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 or fat mass gains. Hypothesis: We hypothesized that LVAD recipients would gain skeletal muscle mass during the first 6 months of LVAD support. Methods: We prospectively recruited 28 adults with systolic HF ±21 days from LVAD implantation. Participants underwent whole-body dual X-ray absorptiometry (DXA) to calculate fat free mass (FFM, representing all lean mass), appendicular lean mass (ALM, lean mass in the arms and legs) and fat mass (FM). DXA was repeated at 3 and 6 months after LVAD implantation (±14 days), with study participation ending after either the 6 month visit or heart transplantation, whichever occurred first. Paired t-testing and mixed effects models were used to evaluate changes over time each for FFM, ALM and FM. Results: The cohort was 86% (24/28) male, with mean age 56 ±12 years and mean BMI 26.6 ±5.5 kg/m 2 at baseline. The median Intermacs class was 2 and duration of HF 50 months. Per European Working Group on Sarcopenia in Older People (EWGSOP) criteria, 41% of participants had muscle wasting at baseline. There was a significant increase from baseline to 3 months and then 6 months of LVAD support for FFM (Fig 1A; baseline: 56.6 ±11.8 kg, n=27; 3 months: 57.9 ±11.3 kg, n=23; 6 months: 62.7 ±11.1 kg, n=17; p-value for change=0.025) and for ALM (Fig 1B; 22.2 ±5.6 kg; 23.2 ±5.0 kg; 25.4 ± 4.5 kg; p<0.001). There was no increase in FM over the same period (p=0.36). Amongst 22 participants with comparison DXAs, 81% had a ≥5% ALM gain by either 3 or 6 months. Conclusions: Among patients with advanced systolic HF and a high baseline prevalence of muscle wasting, there was a significant gain in skeletal muscle mass, as represented by both FFM and ALM, over the first 6 months of LVAD support.