Objective To further clarify the clinicopathological, immunohistochemical and electron microscopic features, and prognostic aspect of a rare esophageal carcinoma, i.e., basaloid squamous carcinoma (BSC).Methods The archival materials of 763 cases of esophageal malignancies (1977-1996) from the Dept. of Pathology, General Hospital of Nanjing Military Region, PLA, were reviewed and sixteen cases (2.1%) of BSC were detected. The clinical and pathological features of these cases were evaluated. Immunohistochemistry (S-P method), histochemical staining, and electron microscope were utilized to further characterize the neoplasm.Results There were 9 males and 7 females, with a mean age of 58 years. The tumors were classified as stage Ⅰ (n=1), stage ⅡA (n=6), stage ⅡB (n=2), stage Ⅲ (n=5), and stage Ⅳ (n=2) according to the criteria of the UICC TNM classification system of malignant tumors (1987). Most neoplasms in our series were located in the middle third of the esophagus. Grossly, they had an appearance similar to that of conventional esophageal carcinoma. Histologically, they showed a typical cytoarchitectural patt of BSC. The most important histologic feature of this tumor was that the carcinoma had a basaloid patt, intimately associated with squamous cell carcinoma, dysplasia, or focal squamous differentiation. The basaloid cells were round or oval in shape with scant cytoplasm. The nuclei showed pleomorphism, hyperchromatin, and numerous mitotic figures. The basaloid cells were arranged mainly in the form of solid, smooth-contoured lobules with peripheral palisading. Comedo necrosis in the islands of basaloid cells, small cystic spaces containing Alcian blue or PAS positive material, and PAS positive hyalinized stroma were frequently found. A panel of immunostains were used for the basaloid component of the tumor with the following results: cytokeratin (Pan),14/16 (+); epithelial membrane antigen, 16/16 (+); Vimentin, 4/16 (+); S-100 protein, 7/16 (+). Carcinoembryonic antigen and smooth muscle actin were negative. Electron microscopic (EM) study of 7 cases revealed that the basaloid cells are poorly differentiated, with a few desmosomes and fibrils and numerous free and polyribosome. The microcystic and intertrabecular spaces identified by light microscopy were seen under EM to be lined by basal membranes and filled with either loose reduplicated or compact globoid basal lamina showing fingerprint-like patt. Of the 11 cases with adequate follow-up, 8 cases died within 2 years, the average survival time being 16.2 months. No cases of stages Ⅱ, Ⅲ, or Ⅳ survived beyond 5 years. The one-year survival rate was 60% and two-year 20%.Conclusion It is indicated that the BSC of the esophagus is a distinct clinicopathological entity with poor prognosis. The cellular differentiation and biologic behavior of the esophageal BSC were assumed to be situated between those of conventional squamous cell carcinoma and small cell carcinoma.