We investigated basal acid output (BAO) by nasogastric aspiration in 28 children with atypical recurrent abdominal pain, defined as subxyphoid or epigastric pain of more than 3 months duration, associated with vomiting, anorexia, failure to thrive, and/or nocturnal awakening. Twenty-one children had normal upper endoscopies with no esophageal mucosal disease on biopsy, and seven had acid-peptic mucosal disease seen either on endoscopy or on esophageal biopsy. Mean BAO in the children with acid-peptic mucosal disease (6.8 +/- 4.3 mEq/h) was significantly higher than the mean BAO in the children with no mucosal disease (2.4 +/- 1.7 mEq/h) (p less than 0.02). Gastric volumes were also significantly greater in the children with acid-peptic mucosal disease than in children with no mucosal disease (p less than 0.001). There was no significant correlation between body weight and either BAO or basal volume output. Three of seven children with acid-peptic mucosal disease had a BAO greater than two standard deviations above the mean for the group with no mucosal disease. Two of these three had a BAO of more than 10.0 mEq/h (defined as gastric acid hypersecretion in adults), and required higher than usual doses of H2 blocker therapy. These results indicate that the values for BAO in children (greater than 2 yr) are comparable to values recorded in adults. Moreover, measurement of BAO may define a subgroup of children with atypical abdominal pain who are hypersecretors of acid and who require higher doses of antisecretory agents in order to achieve an optimal clinical response.