Preoperative chemoradiotherapy has been shown to improve the outcome of patients with esophageal cancer, but because response to this therapy varies, it is desirable to identify in advance individuals who would be unlikely to benefit, in order to avoid unnecessary adverse drug effects. The serum profiles of 84 cytokines and related proteins were determined in 37 patients with esophageal squamous cell carcinoma who received identical neoadjuvant preoperative chemoradiotherapy regimens and underwent surgical resection. Histological response to this therapy was assessed in surgically resected specimens. The serum soluble interleukin-6 receptor (sIL6R) level was significantly higher in 30 patients who failed to achieve a histological complete response (P = 0.005). Multivariate analysis revealed that the increased level of sIL6R was one of several significant independent predictors of an unfavorable outcome (hazard ratio, 2.87; P = 0.017). The increased level of this cytokine in patients who did not obtain a complete response was reproducibly observed in an independent cohort of 34 patients. Esophageal squamous cell carcinoma patients with an increased serum level of sIL6R are predicted to respond poorly to preoperative chemoradiotherapy, therefore, their exclusion from this treatment may be considered. Persistent systemic inflammation is implicated as a possible mechanism of resistance to this therapy.