Incomplete gastric outlet obstruction by a pyloric or prepyloric membrane is rare. A 1-year-old girl had a six-month history of vomiting and failure to thrive. Preoperative roentgenographic findings suggested there was incomplete antral membrane. Since no extrinsic deformity of the pylorus, stomach, or duodenum can be seen at surgery, establishment of the diagnosis preoperatively is very important. Recommended treatment is a Heineke-Mikulicz pyloroplasty with or without incision or excision of the mucosal membrane.