Heterotopic bone formation (osseous metaplasia) is rarely seen in the gastrointestinal tract. It usually occurs in the colorectum in association with benign or malignant epithelial tumors, and most often with mucinous adenocarcinoma. The rectum is the most common location of ossification in the gastrointestinal tract. The pathogenesis of the heterotopic ossification is unknown, but it is probably the result of metaplasia of fibroblasts. We present a case of heterotopic bone formation in a primary rectal adenocarcinoma which was observed in a 73-year-old man.