A case of pigmented hidroacanthoma simplex showing malignant transformation into porocarcinoma is reported. Although no intracellular duct formation could be observed as in benign tumors, ultrastructurally the tumor cells showed characteristics similar to those of eccrine poroma. Many melanocytes were seen dispersed within the tumoral nests. The melanocyte-keratinocyte relationship was found similar to that occurring in melanoacanthoma. Porocarcinoma cells showed ultrastructural features similar to those of benign cells.