Anorectal malignant melanoma, originating from the anorectal canal, is a very aggressive and rare tumor. They account for 0.8-1% of malignant tumors of this region and 0.7% of all malignant melanomas. It is claimed that malignant melanomas of this region originate from the melanocytes or melanocytic proliferation among the stratified squamous epithelium, transitional epithelium of anus or the columnar epithelium of rectum.

In this study, one female and two male patients, aged 62, 62 and 55, admitted to our hospital between years 2001 and 2003 with rectal bleeding and diagnosed as malignant melanoma are discussed. The diagnosis of the rectal biopsy of one patient was undifferentiated carcinoma, and malignant melanoma in the other two, initially. Miles operation was performed. All the tumors were adjacent to the dentate line and they were polypoid masses with partial ulcerations. The greatest diameters of these masses were 5,5, 6 and 10 cm, respectively. Tumor cells showed positive immunoreactivity with HMB45, S100 protein and Melan-A, but immunoreactivity with pancytokeratin was not detected.

Anorectal malignant melanoma is a rare tumor whose morphological features can mimic undifferentiated carcinomas and mesenchymal tumors and especially immunohistochemical examinations of endoscopic biopsy specimens can be helpful in differential diagnosis.