Intestinal obstruction is a common surgical emergency. Transmesenteric hernia is an unusual cause of bowel obstruction that may result in irreversible damage of the bowel and a fatal outcome. Once incarceration of the bowel occurs, strangulation and gangrene follow immediately. The mortality rate associated with this condition is about 15%, but in the presence of gangrene of the bowel, the mortality rate is more than 50%. An accurate preoperative diagnosis of a transmesenteric hernia is very difficult and rarely made. Therefore, in patients with small bowel obstruction, in the absence of a history of previous surgery to suggest adhesions or an external hernia, the possibility of a transmesenteric hernia must be considered. We describe a case with gangrene of a long segment of the small bowel caused by a transmesenteric hernia through a large defect of small bowel mesentery in a child. (Korean J Gastroenterol 2009;53:320-323)