Tumors derived from smooth muscle occur in all segments of the gastro-intestinal tract. They are most common in the stomach and rarest in the colon. Of 28 leiomyomas and leiomyosarcomas, reports of which were collected by Geschickter, only 2 were located in the large intestine and both of these, one benign and one malignant, were found in the rectum. The case report which follows is of interest, therefore, because of the rarity of the lesion. It is also of interest because, owing to certain roentgenologic characteristics, the histologic structure of the tumor was suspected before operation. A man, forty-six years of age, registered at the Mayo Clinic on July 3, 1941, complaining of attacks of pain in the upper left portion of the abdomen. The first attack had occurred more than a year previously. The pain had been accompanied by a temperature of 102· F., a sensation of chilliness, and several watery stools. Relief followed a bowel movement or the passage of gas by rectum. In the three months prior to registration these attacks had become more frequent and the pain had become so severe that the patient was unable to walk upright. At no time was there nausea or vomiting and no blood had been observed in the stools. Marked tenderness and rigidity of the abdomen, especially in the left upper quadrant, were noted on physical examination. Temperature by mouth was 99· F., and the pulse rate ranged from 78 to 88 beats per minute. No lesion was found in the rectum on proctoscopic examination. Laboratory studies brought out the following important facts: (1) the stools contained a large amount of pus, but no ova, parasites, or other organisms of significance; (2) the blood contained 28,300 leukocytes per cubic millimeter. Roentgenologic examination of the chest showed the left side of the diaphragm to be elevated. The roentgenologic examination of the colon (Figs. 1 and 2) was reported as follows: “Tumefactive lesion involving transverse colon near splenic flexure. Bowel is deformed in a semicircular manner. Mucosal aspect apparently preserved. Although exact nature of lesion is not firmly established, the findings are suggestive of intramural extraluminal tumor 6 cm. in diameter, such as leiomyoma, leiomyosarcoma, fibroma, or lipoma.” At surgical exploration Dr. C. W. Mayo found a tumor involving the transverse colon near the splenic flexure. It was adherent to the anterior abdominal wall, to the stomach, and to the jejunum. When opened, it contained foul smelling yellow pus. An extraperitoneal resection was done. The pathologist reported the tumor to be a degenerating, necrotic, infected, submucous leiomyoma of the transverse colon. The roentgenologic characteristics of submucosal, intramural tumors arising in the small intestine have been reported previously (2). They are: (a) preservation of the mucosal relief pattern and (b) production of a rounded deformity of the barium-filled intestinal lumen by a mass which is firmly attached to the bowel.