Inclusion Criteria:

A subject will be eligible for inclusion in this study if all of the following criteria are met:

• Patients with dyspepsia defined as upper abdominal pain or discomfort of at least two months duration.
• Male and female subjects 6 – 17 years of age
• Endoscopic mucosal biopsies revealing at least 20 eosinophils in a high power field on specimens from the duodenum in the absence of other defined duodenal pathology.
• Written informed consent signed by the subject’s legal guardian and by the investigator and patient assent.

Exclusion Criteria:

Subjects will not be eligible for inclusion in this study if any of the following criteria apply:

• A subject who has previously been treated with montelukast.
• Treatment with steroids or oral cromolyn in the four weeks prior to endoscopy or during the study.
• Any history, clinical, and/or biochemical evidence of clinically significant renal or liver disease.
• Treatment with any inducers or inhibitors of CYP3A4.
• Intake of grapefruit juice or oral vitamin E supplementation