The entity, persistent descending mesocolon, although commonly known to European radiologists and frequently noted in their literature, has not previously been discussed in the American radiologic literature (5). This developmental anomaly is characterized by failure of fusion of the mesentery of the left side of the colon with the posterior and lateral parietal peritoneum. A mesentery of varying length persists and may extend from the region of the splenic flexure to the sigmoid colon. This mesentery permits mobility of the descending colon, resulting in considerable variation in its position. The absence of the descending colon, or a portion thereof, from it s usual domain adjacent to the left lateral abdominal wall leaves a space into which all or a part of the small bowel may migrate. This, in addition to the medial position of the descending colon, produces a characteristic roentgen picture on contrast studies of the gastrointestinal tract and occasionally on plain films of the abdomen (Fig. 1). No a...