Congenital smooth-muscle hamartoma.

Congenital smooth-muscle hamartomas are rare, benign tumors of the skin. Since their original description in 1969, 16 case reports have appeared in the literature, with 7 cases reported within the last year. We describe the 17th patient with this lesion and have included a review of the literature. In addition, we describe three morphologic types of smooth muscle cells-the pale cells, dark cells, and intermediate cells-found on electron microscopy, and the features that support our belief that these may reflect different stages of maturity of the smooth-muscle cells. In addition to the smooth muscle, bundles of nerve fibers (both unmyelinated and myelinated) appear to be an intrinsic part of the lesion, as are the prominent vellus hairs. Congenital smooth muscle hamartoma thus appears to be an organoid nevus.