BACKGROUND: Loss of skeletal muscle is predictive of a poor prognosis in patients with various malignant lesions. Our aim was to determine whether changes in skeletal muscle after neo-adjuvant therapy (NAT) predict prognosis in patients with esophageal squamous cell carcinoma (ESCC) undergoing esophagectomy.PATIENTS AND METHODS: The cross-sectional areas of the psoas muscles were measured on computed tomographic images collected at the initial visit, preoperatively and postoperatively in 84 patients. The psoas muscle index (PMI) was calculated by normalizing the cross-sectional areas to the patients' heights.RESULTS: Low PMI at the initial visit was not associated with a poor prognosis. The majority of patients showed decreased PMI after NAT and surgery. The group in which the post-NAT PMI decreased had poorer overall survival than group without PMI decrease (p=0.025).CONCLUSION: Decreased PMI correlates well with a poor prognosis in patients with ESCC. Changes in PMI over a period of time may have greater sensitivity when evaluating prognosis than the PMI at any single time point.