The patient is a 13-year-old female who had had a patchy lesion on her back since birth which increased in size proportional to her age. When she visited our hospital at the age of 13, the lesion was a 30 x 43 mm, slightly depressed, well defined, irregularly shaped, atrophic plaque. Differential clinical diagnoses at that time included macular atrophy, congenital melanocytic nevus, and mastocytosis. Histopathologically, the lesion was characterized by patchy and scattered infiltrates of oval to spindle cells throughout the dermis extending into the subcutaneous fat tissue. Nerve fibers were found in each of the infiltrates. The tumor cells had oval nuclei with relatively abundant cytoplasm and were similar to nevus cells. They were negative for markers such as S-100 protein, neuron specific enolase, epithelial membrane antigen, HMB-45, Leu 7, Desmin, and alpha smooth muscle actin. However, the histopathological findings suggest that the lesion is a melanocytic or neurogenic tumor, possibly a congenital melanocytic nevus or a cellular neurothekoma, though a definite diagnosis could not be made.