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It is very difficult to distinguish primary cutaneous carcinomas correctly from metastatic carcinomas. In addition, primary cutaneous lymphomas closely resemble various pseudolymphomas. Apocrine carcinomas and extramammary Paget's diseases possessed cytokeratin20(CK20)-/gross cystic disease fluid protein(GCDFP)15+ immunoreactivity, while pagetoid phenomena from metastatic transitional cell carcinomas and rectal carcinomas showed CK20+/GCDFP15- immunoreactivity. Primary cutaneous signet-ring cell carcinomas were positive for both CK20 and GCDFP15. CK20-positive primary cutaneous carcinomas should include primary cutaneous signet-ring cell carcinoma in addition to Merkel cell carcinoma. Primary cutaneous B-cell lymphomas including marginal zone B-cell lymphoma demonstrated light chain restriction immunohistochemically. In various pseudolymphomas including acral pseudolymphomatous angiokeratoma of children, there was a mixture of kappa-positive and lambda-positive cells, although kappa-positive cells predominated. These results indicate that immunohistochemical analysis is very useful in diagnosing primary cutaneous malignant tumors.