Two cases of malignant hidroacanthoma simplex are reported. The first case was a 73-year-old man with a tumor on his left thigh of six months duration. The tumor was brown black keratotic, measuring 1.0×1.0cm. The other case was a 86-year-old man with a tumor on his back of four months duration. The tumor formed a grouped, red papular lesion measuring 1.5×1.5cm. On histological examination, the tumors in both cases showed a well-defined, intraepidermal nest formed by small, cuboidal cells. Some of the cells had atypical nuclei and abundant cytoplasm, but dermal invasion of the cells was not seen. The epidermis was acanthotic and hyperkeratotic. Differentiation of the tumor cells to small ductal structures and cystic areas were also seen in both cases.