Eruptive hidradenoma, a rare type of syringoma, was studied histologically, histochemically, and electron microscopically in two patients. In spite of its peculiar clinical differences, such as occurrence of numerous lesions with limited distribution to anterior surfaces of the body, it showed features identical to the eyelid type syringoma. Direct connections were found between the lesions and epidermis and between individual lesions, but not between the lesions and underlying eccrine secretory segments. Also, cystic or tubular spaces were lined with an "immature eccrine intra-epidermal type" of epithelium which possessed a periluminal tonofilament band, lysosomes, keratohyaline granules, and often underwent complete keratinization. No secretory epithelium was found in the lesion. Dermal histiocytes with typical Langerhans cell granules and collagen fiber bundles were seen in parenchyma. It seemed that the lesion was undergoing constant destruction and regeneration.