A 41-year old mentally retarded patient presented acutely with a 3 day history of vomiting and absolute constipation. Intestinal obstruction was diagnosed following an abdominal x ray. At laparotomy, a left paraduodenal hernia was present, without incarceration of small bowel. The herniated loops were reduced and the hernia orifice closed. The anatomy, treatment and importance of considering this uncommon diagnosis when examining a patient with acute small bowel obstruction are discussed.