A congenital arrector pili hamartoma is a neoplasm that presents as multiple or solitary dermal nodules in a dermatomal distribution. To elucidate and clarify its histogenesis, a lesion derived from a 3-year-old boy was studied by light microscopy, indirect immunofluorescence, using antibodies against basal lamina constituents and against interstitial matrix components, and electron microscopy. In addition, a rabbit antibody specific for bovine smooth-muscle myosin was used. The antibodies against the basal lamina components and fibronectin all showed an intense perimysial fluorescence that ensheathed and surrounded individual leiomyocytes. Anti-smooth-muscle myosin exhibited intense cytoplasmic fluorescence. Furthermore, electron microscopy showed fusiform cells with abundant myofilaments, dense bodies, and pericellular basal lamina as seen in smooth muscle. These studies suggest the probable origin of this hamartoma from pili arrector muscle and could be used as an adjunct in histopatological diagnosis.