© 2019 European Society for Clinical Nutrition and Metabolism Background and aims: Malnutrition and sarcopenia are common nutrition (-related) disorders in patients with COPD and are associated with negative health outcomes and mortality. This study aims to correlate ultrasound measured rectus femoris size with fat-free mass and muscle function in patients with COPD. Methods: Patients with COPD, at the start of a pulmonary rehabilitation program, were asked to participate in this study. Rectus femoris (RF) size (thickness in cm, cross-sectional area [CSA] in cm 2 ) was determined by ultrasound. Fat-free mass index (FFMI in kg/m 2 ) was estimated with bioelectrical impedance analyses, using a disease-specific equation. Handgrip strength (HGS) was measured in kilograms and the five times sit to stand test (in seconds, higher scores indicating decreased strength) was performed to assess leg muscle power. The Incremental Shuttle Walk Test (ISWT, in m) was used to assess maximal exercise capacity. Results: In total, 44 patients with COPD (mean age 59.8 ± 8.6 years, 43% male, median FEV1%pred 37 [IQR = 23–52]) were included. Greater RF-CSA and thickness were associated with higher FFMI (r = 0.57, p < 0.001; r = 0.53, p = 0.003, respectively) and HGS (CSA r = 0.58, p < 0.001, thickness r = 0.48, p = 0.009). No significant correlations between RF-thickness, CSA, and leg muscle power were found (r = −0.33, p = 0.091; r = −0.35, p = 0.073, respectively). Furthermore, no correlation between RF size and maximal exercise capacity was observed (thickness r = 0.21, p = 0.297, CSA r = 0.22, p = 0.274). Conclusions: This exploratory study shows that in patients with COPD, rectus femoris size is moderately correlated with FFMI and HGS. Future studies should focus on the role of ultrasound in evaluating nutritional status.