The CT findings of invaginated mesentery in adult intussusception were analyzed in 12 patients (three patients with four small bowel intussusceptions and nine patients with nine large bowel intussusceptions). In all small bowel intussusceptions, the neck was located near the superior mesenteric artery and vein (SMA & SMV) and showed a medial opening, and the vessels in the neck showed a continuity to the left side of SMA & SMV. In most of the large bowel intussusceptions, the neck was located distant from SMA & SMV, and the vessels in the neck did not show continuity to SMA & SMV. The difference in CT findings between small and large bowel intussusception is attributed to the difference in mesenteric anatomy; the small bowel and its mesentery move freely in the peritoneal cavity, whereas the large bowel is partially fixed to the retroperitoneum.