Eccrine syringofibroadenoma is a rare benign skin tumor, which usually develops on the extremities of elderly persons. We performed immunohistochemical and ultrastructural studies of a typical case of eccrine syringofibroadenoma that developed on the left heel of a 58-year-old man. The tumor consisted of anastomosing thin epithelial strands connected to the epidermis. There were many ductal or cystic structures, and their luminal cells were strongly positive to antibodies against carcinoembryonic antigen and epithelial membrane antigen. Filagrin and involucrin immunoreactivities were also detected in some cells surrounding the ducts. Keratins K1 and K10, co-expressed in the peripheral cells of normal acrosyringia, were colocalized in small cell clusters. Ultrastructurally, intracellular duct formation characteristic of developing acrosyringia was observed. Tumor cells containing globular keratohyaline granules with various electron densities were seen around some ductal structures. In these areas, keratinization took place without lamellar granule formation or prominent cornified cell envelope assembly. These results suggest acrosyringial differentiation of this tumor.