The paper presents a clinical case of a rare stromal colon tumor in a patient previously operated for breast cancer. The patient with a rapidly growing abdominal tumor of unknown origin underwent tests in the department of coloproctology. According to ultrasound and CT examina-tion, the lumpy formation with large size and low mobility, compressing the surrounding organs and probably coming from the colon, has been revealed. The removal of the tumor coming from the sigmoid colon with segmental resection anastomosis was performed. The postoperative period was uncomplicated, patient discharged from the clinic 7 days after surgery. Immunohistochemical examination of the tumor showed Gastrointestinal Autonomous Neurogenic Tumor (GANT) – a variant of GIST. Surgery for rare mesenchymal tumors is better in the coloproctologi-cal units with sufficient skills of surgeons. The multidisciplinary approach is necessary in such cases.