[A case of malignant clear cell hidradenoma of the skin presumably arising from adenoma].

A left parietal skin tumor showing rapid enlargement in a 33-year-old woman was excised. The cut surface of the skin revealed a whitish tumor 4 cm in diameter associated with a cyst formation. Histologically, the whitish tumor was mucinous carcinoma in which large clear cells were conspicuous. Because nests of clear cell hidradenoma (CCH) were seen in the cyst wall, this carcinoma was diagnosed as malignant CCH presumably arising from a part of benign CCH. Malignant CCHs are thought to be malignant from their onset. Those proved to have developed from benign CCHs like the present case are extremely rare.