Dermal melanocytosis is characterized by the presence of ectopic melanocytes in the dermis. The most common forms include the Mongolian spot, blue nevus, nevus of Ota, and nevus of Ito. Some types of dermal melanocytosis do not fit into any of these morphologic categories, however. Our case demonstrated an extensive amount of uniform deep blue patches of nevi with unilateral distribution on the left face, neck, chest, shoulder, and back. On histopathologic examination, a number of elongated melanocytes scattered throughout the dermis were found. We herein report a case of congenital unilateral dermal melanocytosis. (Ann Dermatol 22(4) 460~462, 2010)