Background: Weight loss during chemotherapy has not been exclusively investigated. Macrophage inhibitory cytokine-1 (MIC-1) might play a role in its etiology. Here, we investigated the prognostic value of weight loss before chemotherapy and its relationship with MIC-1 concentration and its occurrence during chemotherapy in patients with advanced esophageal squamous cell carcinoma (ESCC). Materials and Methods: We analyzed 157 inoperable locally advanced or metastatic ESCC patients receiving first-line chemotherapy. Serum MIC-1 concentrations were assessed before chemotherapy. Patients were assigned into two groups according to their weight loss before or during chemotherapy:>5% weight loss group and ${leq}5%$ weight loss group. Results: Patients with weight loss>5% before chemotherapy had shorter progression-free survival period (5.8 months vs. 8.7 months; p=0.027) and overall survival (10.8 months vs. 20.0 months; p=0.010). Patients with weight loss >5% during chemotherapy tended to have shorter progression-free survival (6.0 months vs. 8.1 months; p=0.062) and overall survival (8.6 months vs. 18.0 months; p=0.022), and if weight loss was reversed during chemotherapy, survival rates improved. Furthermore, serum MIC-1 concentration was closely related to weight loss before chemotherapy (p=0.001) Conclusions: Weight loss both before and during chemotherapy predicted poor outcome in advanced ESCC patients, and MIC-1 might be involved in the development of weight loss in such patients.