We report a case of a 30-year-old man who presented with severe debilitation, anemia and diarrhea over two months. Colonoscopy revealed many (>100) polyps (familial adenomatous polyposis syndrome). Abdominal CT scan showed a large mass at the left upper abdomen in conjunction with the splenic flexure. Total colectomy with mesenteric mass and adjacent small bowel removal and ileoanal pouch was performed. Examination of the resected mesenteric mass showed a gastrointestinal stromal tumor (GIST) with scarce mitosis and infiltration of the adjacent small bowel. We describe for the first time in medical literature the coexistence of familial adenomatous polyposis syndrome and GIST in a 30-year-old man.