A single poroid neoplasm composed of three histologically distinct lesions (hidroacanthomas simplex, eccrine poroma, and dermal duct tumor) is reported. Comparative histologic, histochemical, and electron-microscopic studies revealed that each tumor subtype contained varying proportions of poroid cells, clear cells, and cuticular cells. The major component of all three neoplasms was poroid cells, which, under the electron microscope, were characterized by a few, small, poorly developed desmosomes, and were histochemically characterized by a positive succinic dehydrogenase reaction. The dermal duct tumor was cultured, and showed similar histochemical findings to the in vivo poroid cells. These results suggest that poroid cells plays the most important role in the histogenesis of these three neoplasma.