The position of the duodenojejunal flexure was determined in 37 out of a group of 49 consecutive infants presenting with intussusception. The flexure was in a normal position in 19, at the midline in 3, and in an abnormal position in 15. The caecum was unfixed in all of the 41 infants subjected to operation. It is concluded that lack of normal rotation and fixation of the intestine is an important factor in the aetiology of idiopathic intussusception of infants.