Apoeccrine glands were found in 13 of 57 specimens of nevus sebaceus. The glands showed the basic structure of eccrine glands, but differed in that a part of the secretory duct was lined by characteristic apocrine epithelium. The apocrine alteration varied from slight apocrine changes in a few secretory cells to an almost complete alteration of the secretory epithelium in other glands. The association with changes in the excretory ducts of rare glands suggested a transformation of eccrine into apocrine glands. It was nevertheless considered as most likely that a majority of the apocrine glands in nevus sebaceus had developed in the "classic" way in association with pilosebaceus structures. In nevus sebaceus, apoeccrine glands appeared to represent eccrine glands with a variable degree of apocrine metaplasia that could result in complete transformation rather than the formation of a separate category of specialized sweat glands. The capacity of eccrine epithelium to transform into apocrine epithelium is why difficulties are experienced in the precise categorization of sweat gland neoplasms into eccrine and apocrine tumors.