18F-FDG maximum intensity projection image. Colonoscopy examination of a 51-year-old man, whose only complaint was severe rectal bleeding, revealed multiple, large necrotic polypoid lesions in all colonic segments. Excisional biopsy has been performed from the sigmoid region. Histopathologic findings and immunohistochemistry analyses including S-100, HMB45 and vimentin were positive and all these findings strongly suggested colonic malignant melanoma. Ophthalmologic, dermatologic and ear-nose-throat examinations were negative for primary melanoma or any melanocytic lesion, thus the case was diagnosed as a colonic malignant melanoma. Melanomas within the gastrointestinal (GI) tract are usually metastatic in origin (1). However, some colonic melanomas are true primary tumors. The probable genesis of such tumors involves a concept of “ectodermal differentiation” - that ectodermal cells are capable of differentiation into multiple cell lines and may variably migrate into the colon during embryologic stages to develop into melanocytes (2). Primitive stem cells localized within the GI tract wall may also give rise to heterotopic melanocytes in the colon (3).