Poorly differentiated adenocarcinoma of colorectal carcinoma (CRC) is a rare condition with poor prognosis. Here, we describe the case of a 69-year-old man who underwent laparoscopic low anterior resection and was pathologically diagnosed with stage IIIA CRC. At 10 months post-operation, he developed fever and loss of appetite. Laboratory examination revealed brin degradation products > 120.0 µg/dL and D-dimer > 60.0 µg/dL. Bone marrow (BM) examination showed malignant epithelioid in ltrate with CK20 and CDX2 expression, leading to a diagnosis of disseminated carcinomatosis of bone marrow, which is rare in CRC and indicates widespread disease is throughout the body. Furthermore, immunohistochemistry indicated the predominant expression of receptor activator of nuclear factor κB ligand (RANKL) in CRC and BM tissues. Thus, RANKL expression, which is known to indicate metastatic behavior of cancer cells, in CRC may play a critical role in promoting osteoclast formation, which has been associated with the pathogenesis of BM lesions.