The embryonic stem cell factors Oct3/4 and Sox2 are essential for pluripotency and self-renewal of embryonic stem cells. Cancer cells, especially in poorly differentiated or undifferentiated tumours, have been characterized by many phenotypic traits similar to undifferentiated embryonic cells, indicating that Oct3/4 and Sox2 may be expressed in solid tumours. With the methods of real-time PCR, Western blotting and immunocytochemistry/immunohistochemistry, the expression of these two genes in the esophageal squamous cancer cell lines Kyse70, Kyse140 and Kyse450 were characterized, in addition to a virus-transformed "normal" esophageal epithelial cell line, Ket-1A. Both Oct3/4 and Sox2 were variably expressed in the cancer cell lines, but were either negative or very weakly expressed in the normal cell line. Further examinations in a series of 162 consecutive esophageal squamous cancer patients showed that 17.90% and 22.84% of the tumours highly expressed Oct3/4 and Sox2 proteins, respectively, and the expressions of these two factors were significantly associated with higher histological grade and poorer clinical survival. Since the function of pluripotency and self-renewal of these factors has been characterized in human embryonic stem cells, these data may indicate that the expression of these factors enables the tumours to have higher degree of stemness tumour cells, which in turn results in poorer clinical outcome for patients with esophageal squamous cell carcinomas.