An 82‐year‐old Caucasian man developed an ulcerated mass on the anterior mandibular gingiva. Five years previously he had been treated for a Merkel cell carcinoma (MCC) on his right cheek. Histopathologic examination showed small tumor cells with scanty cytoplasm, suggestive of malignancy. Immunohistochemical studies were performed with the use of nine antibodies. S‐100 protein and leukocyte common antigen were helpful in ruling out melanoma and lymphoma. Pronounced reaction was shown for cytokeratin 20. a new histodiagnostic marker whose expression is almost entirely confined to Merkel cells, the gastric epithelium, and urothelium. The tentative diagnosis of metastasis of MCC was confirmed. Immunohistochemical studies are useful diagnostic aids in the establishment of the diagnosis of Merkel cell carc