PURPOSE: To establish whether Sphingosine-1-phosphate (S1P) and sphingosine kinase 1 (SphK1) contribute to lymph node metastasis in esophageal squamous cell carcinoma. METHODS: Immunohistochemical analysis of SphK1 expression was performed using a tissue microarray containing 177 thoracic squamous cell esophageal cancer specimens resected at surgery, to investigate the association between intratumoral SphK1 expression and lymph node metastasis. Serum S1P levels and intratumoral SphK1 mRNA and protein expression were also evaluated in mice with vs. mice without lymph node metastasis in a murine lymph node metastasis model. RESULTS: Among 177 esophageal cancer patients, 127 (72%) were defined as being SphK1-positive. In univariate and multivariate analyses, SphK1 expression status was a significant factor contributing to lymph node metastasis and poorer 5-year overall survival. In the murine lymph node metastasis model, there was no difference in tumor volume or weight between the lymph node metastasis-negative and lymph node metastasis-positive groups. However, levels of SphK1 mRNA and protein and serum S1P levels were all much higher in the metastasis-positive group. CONCLUSIONS: S1P/SphK1 may be novel targets for inhibiting lymph node metastasis in esophageal squamous cell carcinoma, and may provide the basis for a therapeutic strategy to suppress lymph node metastasis.