A 79-year-old woman visited a previous hospital with a complaint of general fatigue. The patient was diagnosed with cecal cancer with multiple liver metastases and lymph node metastases on colonoscopy, abdominal ultrasonography and CT scan, and was referred to our division for treatment. Based on the diagnosis of non-curative colonic cancer, we planned to perform systematic chemotherapy after local surgical treatment. We performed an ileocecal resection, and the specimen showed poorly differentiated adenocarcinoma with mutation in the BRAF oncogene. After the surgical treatment, the tumor grew rapidly and the patient died from cancer on the 19th postoperative day without having the opportunity to undergo chemotherapy. Multiple genetic and epigenetic alterations in oncogenes and tumor suppressor genes are involved in the process of colorectal carcinogenesis. Some of the alterations have been identified as predictive and prognostic biomarkers. A mutation in the BRAF oncogene was reported to be associated with a very unfavorable prognosis in colorectal cancers. Some of the cases with rapid progression are suggested to have the BRAF oncogene mutation. According to our experience, chemotherapy before surgical treatment might improve the prognosis of cases with the BRAF mutation.