Primary anorectal malignant melanoma: report of a case.

Primary anorectal malignant melanoma is a fairly uncommon but highly malignant disease. This disease is sometimes mistaken for such benign conditions as either a hemorrhoid or rectal polyp. We herein describe a case of early primary malignant melanoma of the anal canal. In this case, magnetic resonance (MR) imaging was found to be useful for diagnosing the melanotic melanoma. We especially emphasize the usefulness of a fat-saturation MR image in distinguishing melanotic melanoma from other rectal tumors.