The occurrence of leiomyoma of the colon is uncommon. Most of these lesions are clinically silent and are found incidentally during laparotomy or endoscopic procedures for unrelated conditions. Symptomatic leiomyomas of the colon are encountered less frequently, with only sporadic reports in the literatures. We describe a heretofore unreported case of a large extraluminal leiomyoma of the sigmoid colon presenting as massive lower gastrointestinal hemorrhage. Because it was extraluminal in position, it was difficult to make an accurate diagnosis endoscopically and the condition was easily misdiagnosed as angiodysplasia of the colon until CT scan results were seen. Although rare and benign in nature, leiomyoma of the colon may cause life-threatening complications that require emergency surgery and should be included in the differential diagnosis of lower gastrointestinal hemorrhage.