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Summary: Background and aims: Muscle mass is an important component of nutritional status. At intensive care unit (ICU) admission, ultrasound is an important tool to assess the muscle mass, but there are currently no established cut-off values. Therefore, we retrospectively analyzed cut-off values for an ultrasound-based muscle mass assessment at ICU admission. Methods: We retrospectively analyzed data collected in an observational study. These data included a muscle mass assessment of lower limb thickness and cross-sectional area (CSA), upper limb thickness and CSA, and diaphragm thickness at ICU admission. Skeletal muscle mass was calculated from computed tomography (CT) imaging of the third lumbar vertebra for comparison. Cut-off values were estimated from the relationship between the ultrasound-based muscle mass assessment and CT-derived skeletal muscle index at 7.0 kg/m2 in males and 5.4 kg/m2 in females. Results: Lower limb, upper limb, and diaphragm assessments included 89, 52, and 79 patients, respectively. The age was 70 ± 13 years and body mass index was 22.2 ± 4.4 kg/m2. Cut-off values were 22.5 mm (male) and 19.0 mm (female) for femoral thickness and 5.0 cm2 (male) and 4.5 cm2 (female) for rectus femoris CSA. Cut-off values were 26.9mm (male) and 20.6 mm (female) for upper limb thickness and 6.5 cm2 (male) and 4.0 cm2 (female) for biceps brachii CSA. Cut-off values were 1.8 mm (male) and 1.5 mm (female) for diaphragm. Conclusion: We proposed cut-off values for ultrasound-based muscle mass assessment at ICU admission. Further studies are needed to validate these cut-off values.