The vast majority of anal fistulas do not undergo malignant transformation. However, there is a possibility of cancerous changes if an anal fistula that has repeatedly recurred over a long period is not treated aggressively, or if the inflammation of the fistula tract is severe. When an anal fistula undergoes malignant transformation, the secretion from the local fistula tract increases and is accompanied by a foul smell, and there may even be ulceration of the local fistula tract mucosa. Further surgical procedures, such as fistulotomy with seton placement, are required, and the excised fistula tract suspected of high-grade malignant transformation should be sent for pathological examination. If malignant transformation is confirmed, further pelvic MRI examinations are necessary to determine whether there is local and surrounding lymph node metastasis.