Skin biopsy specimens from 12 elderly patients with Merkel cell tumors were investigated. Conventional light microscopy and immunohistochemical techniques were used. All of the tumors had similar morphologic features. Immunoreactivity for neuronspecific enolase, gastrin, calcitonin, and epithelial membrane-like antigen was demonstrated, and both neurofilaments and keratin filaments were observed. The immunohistochemical findings supported a Merkel cell origin for these Merkel cell tumors. The co-expression of neuroendocrine and epithelial markers in Merkel cell carcinomas is suggestive of neuroendocrine differentiation in a neoplasm of epithelial origin. Merkel cell carcinomas share many characteristics with neuroendocrine tumors of the bronchopulmonary and gastrointestinal tracts. All of these neoplasms may originate from cells of similar types that are present in several organs.