Esophageal small cell carcinoma (SmCC) is a rarer, more highly aggressive, and more rapidly growing neoplasm than esophageal squamous cell carcinoma (SqCC). SmCC and SqCC also differ in terms of chemotherapy of choice, response to therapy, and prognosis. Accordingly, it is important to differentiate the 2 carcinomas. We studied the histology and immunohistochemical profiles of 6 cases of esophageal SmCC to elucidate the correct diagnosis of this tumor. We performed immunohistochemical analysis antibodies against cytokeratins (CKAE1/AE3, CKCAM5.2, CK34betaE12, CK7, CK8, CK10/13, and CK19), epithelial membrane antigen (EMA), neural cell adhesion molecule (NCAM; CD56), neuron-specific enolase (NSE), chromogranin-A, S-100 protein, carcinoembryonic antigen (CEA), E-cadherin, thyroid transcription factor-1 (TTF-1), and p53. In 3 of the 6 SmCCs, heterogeneous components of in situ or invasive SqCC were observed. SqCC was found in the mucosa adjacent to the main SmCC, and the boundary between SmCC and SqCC was distinct, with no transitional features. Staining for NCAM, NSE, and chromogranin-A was positive in SmCCs, but negative in SqCCs. Both SmCCs and SqCCs were positive for CKAE1/AE3, CKCAM5.2, CK8, and EMA, but only SqCCs were positive for CK34betaE12 and CK19. Moreover, SmCCs containing SqCC components were positive for CEA and E-cadherin, whereas SmCCs without SqCC were negative. Our study suggests that NCAM and NSE are useful markers in diagnosing esophageal SmCC, and CK34betaE12 and CK19 are useful for differentiating SqCC components from SmCC.