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Here we report the case of a 50-year-old woman with adenocarcinoma of the colon, showing heterotopic ossification. The patient was referred to our hospital for investigation of anemia secondary to occult gastrointestinal blood loss. By colonoscopy, an irregular polypoid mass was found in the ascending colon. A biopsy of the lesion revealed moderately to poorly differentiated adenocarcinoma with heterotopic ossification. A right hemicolectomy was done and revealed areas of heterotopic bone within the tumor, but no ossification was evident in the metastatic lesions within the mesenteric lymph nodes. The formation of heterotopic bone in gastrointestinal tumors is rare and its exact mechanism is unknown. Immunohistochemical localization of bone morphogenetic proteins (BMP), known to be primary inducers of new bone formation, was determined. BMP-5 and -6 were prominent in the cytoplasm of tumor cells, and they stained weakly in osteoblast-like cells adjacent to newly formed bone. Cytoplasmic staining for BMP-2 and -4 was weak in tumor cells, osteoblast-like cells, and stromal fibroblast cells. BMP may play an important role in heterotopic ossification in colon adenocarcinoma.