Abstract Body: [Introduction] In acute stroke treatment, it is known that complications of muscle mass loss, such as sarcopenia, increase mortality and delay functional recovery. However, it is often difficult to measure muscle mass in the acute stroke phase. On the other hand, it has been reported that masseter muscle thickness reflects the total body muscle mass. We therefore investigated whether it is possible to determine muscle mass loss by measuring the masseter muscle volume on computed tomography (CT) images, which are taken at the same time as the stroke diagnosis. Objective: To compare the relationship between muscle mass (Skeletal Muscle mass Index: SMI) measured by body composition in patients in the acute phase of stroke and the masseter muscle area captured by head CT images. Methods: The relationship between the masseter muscle area measured from the first head CT image taken for stroke diagnosis and body composition data within 24 hours after admission was investigated in 385 patients (119 women and 266 men) admitted to the Stroke Care Unit of our hospital. [Results] The correlation coefficient between SMI and masseter muscle area was 0.46 in women and 0.44 in men. In women, SMI <5.7kg/m², a diagnostic criterion for sarcopenia, was detectable when the masseter muscle area was <145 mm². (Sensitivity 89%, specificity 85%, AUC 0.929) In men, SMI <7.0kg/m² was detectable with a masseter muscle area <180 mm². (Sensitivity 86%, specificity 83%, AUC 0.912) [Conclusion] It is possible to determine the risk of muscle mass loss from head CT images taken in the acute phase of stroke. This method may assist in determining the rehabilitation treatment strategy and nutritional dosage for stroke patients.