Forty-three control patients (neonate to 17 years old) were studied by upper gastrointestinal series to determine the position of key duodenal landmarks and the mobility of the duodenojejunal flexure with manual displacement. These results were compared with the duodenal positions of 35 children of similar ages with surgically documented malrotation. Nine criteria were identified as a useful means of detecting subtle abnormalities of duodenal position. The normal duodenojejunal flexure was found to be readily displaceable in neonates and could be pushed to the right of the spine in over two-thirds of patients less than 4 months old. Over 4 years of age, mobility was very limited. A mobile duodenum discovered on fluoroscopic examination or by positioning of a transpyloric feeding tube should not be considered indicative of malrotation in infancy.