Decrease in lower esophageal sphincter (LES) pressure and frequent acid reflux were observed in 3 of 5 adult patients with rumination syndrome confirmed by antroduodenal manometry. The role of the LES in the rumination syndrome is unclear but decreased LES pressure could be a determining factor. Antroduodenal manometric studies should therefore be performed during ordinary gastroesophageal reflux in order to determine the diagnostic value of antroduodenal manometry in the rumination syndrome.