The clinical findings in 12 patients who exhibited marked to-and-fro peristalsis in the duodenum with extrinsic compression of the third part and proximal dilatation at barium meal examination are described. In nine cases the compression was thought to be due to the superior mesenteric artery and its mesentery. In two of these cases there was associated duodenal ulceration and in another patient there was evidence of chronic idiopathic intestinal pseudo-obstruction. In the remaining three cases, the duodenal compression was caused either by tumour or malignant glands in the root of the mesentery. It is emphasised that vigorous to-and-fro peristalsis in the proximal duodenum seen at fluoroscopy is an abnormal radiological feature which is associated in most cases with significant pathology.