This is a single site study of the effect of montelukast on eosinophil and mast cell counts and activation in the lining of the duodenum in 24 children with dyspepsia in association with duodenal eosinophilia in association with measuring the concentration of the medication in the lining of the duodenum. Patients will be endoscoped with biopsies obtained from the duodenum as part of routine clinical care. Participants in the study will then receive montelukast daily and the endoscopy with biopsies will be repeated on day 21 to measure cell counts and activation and tissue montelukast levels. Cell counts and measures of activation will be compared to pre-treatment levels.