A 48-year old man suffered from intermittent hematochezia and abdominal cramping over a period of four weeks. A colonoscopy revealed a 4.5 cm semi-pedunculated tumor in the transverse colon. Using a 2-channel colonoscope, the tumor was successfully removed with an electro-surgical snare after normal saline submucosal injection. Microscopic examination revealed it to be a smooth muscle tumor without mitosis. The patient recovered well, and did not have a residual tumor in a follow-up colonoscopy. We found no reports of a colonic leiomyoma larger than 3 cm which was completely removed by a colonoscope. According to the case presented here, intraluminal colonic leiomyoma can be completely resected with skillful manipulation of a colonoscope, even if the tumor is as large as 4.5 cm. Successful endoscopic polypectomy of colonic leiomyoma reduces the cost of treatment and eliminates unnecessary surgery.