A case of malignant hidroacanthoma simplex of the anterior aspect of the right ankle in a 75-year-old man is reported. A specimen obtained in 1987 showed the features of benign hidroacanthoma simplex, whereas that taken in 1991 revealed malignant transformation. We performed immunohistochemical studies on these two specimens, and they suggest this tumor derives from the outer cells of intraepidermal ducts. The proliferating cell nuclear antigen labelling index and argyrophil nucleolar organizer regions of these two specimens were also compared and correlated with malignant transformation.