Abstract: Objective To study clinicopathological feature of anorectal malignant melanoma and the role of immunohistochemical stain in diagnosis.Method 10 cases of anorectal malignant malanoma were studied.Expression of HMB45, S-100, Vimetin were detected by immunohistochemistry with S-P method.Result 1 case is malignant melanoma.9 cases were misdiagnosed as other diseases.Clinicopathologic features:7 cases with essential epitheliod cell, 2 cases with mainly spindle cell, 1 case with small lymphocyte-like cell.All 10cases of anorectal malignant melanoma were positive to HMB45, S-100 and 9 cases were positive to vimetin and 1 case were positive to CK, but negative to LCA.Conclusion The faeces with fresh blood, no evident mucus.The masses were nodular, fungiform or ulcerative.Histopathologic observation supported the idea that this tumor developed from the melanocytes.10 cases at the basal-layer of the epithelium.Immunohistochemistry proved that melanocytes originated from the neural crest.Immunohistochemical stain HMB45, Vimetin S-100 were helpful for diagnosis of malignant melanoma.

Key words: Melanoma, Anarectum, Immunohistochemistry, Clinicopothology