Hidroacanthoma simplex is a benign tumor of the skin originating from or showing differentiation to the sweat glands. It grossly resembles seborrheic keratosis of Bowen's disease and histologically shows intraepidermal focal growth of epithelial cells. Malignant transformation of this tumor is rare. We report a case of pigmented hidroacanthoma with malignant transformation in a 67-year-old woman. There was a 20-year history of a skin lesion on the right thigh, which first appeared as a small verrucous papule, progressed to a dark-brown colored patch, and then to a pigmented plaque. Histologically, the primary tumor was composed of small squamoid cells with marked cellular atypia. Most of the tumor cells were located in the epidermis. Immunohistochemically, the cytoplasm of some tumor cells showed a positive reaction for epithelial membrane antigen, but not for either carcino-embryonic antigen or the S-100 protein.