The benign tumors of sweat glands have been the subject of continuing controversy for many years. Our observations support the concept of an eccrine sweat gland origin for one of these, the dermal eccrine cylindroma. Under the light miscroscope numerous duct-like structures found within tumors were noted to have cuticular borders closely resembling those of the normal eccrine excretory duct, and transitions occurring between the two have been reported.3 With the electron microscope, similarities in ultra-structure were also found between the tumor and the normal eccrine gland. These similarities were seen in the cuticular borders of the duct-like structures, the desmosomes of connecting cells, the dermal attachment plates abutting the dermis, and the secretory vacuoles within tumor cells. By histochemical techniques, the acid mucopolysaccharide present within intercellular spaces between tumor cells was found to be identical with that secreted by mucoid cells of normal eccrine sweat glands and may represent an acidic mucopolysaccharide protein complex of the sialomucin type. Pallavicini, Gabriel, di Sant' Agnese, and Buskirk28 have recently isolated a carbohydrate-protein complex containing sialic acid from the secretion of human eccrine sweat glands. Significantly, no histochemical or structural evidence for the presence of myoepithelial cells was found which might have supported an apocrine origin. Finally, the results of x-ray therapy support our concept of an eccrine sweat gland origin for the dermal eccrine cylindroma. The similarity in cell type and lobular patterns in some areas between eccrine spiradenoma and dermal eccrine cylindroma, and the resemblance of microscopic sections of x-ray treated lesions of cylindroma to syringoma,8 all suggest a common eccrine origin for these three tumors. The large-cell sweat gland adenoma8 and eccrine poroma29 could also be listed to complete a spectrum of overlapping tumors of eccrine origin which involve various levels of the dermis and epidermis.