Inclusion Criteria:

• Physician-diagnosed asthma

• At least one of the following lung function criteria must be documented in the year prior to study entry:

  1. Bronchial hyperresponsiveness confirmed by 12% or greater improvement in FEV1 post-bronchodilator, or
  2. Methacholine PC20 less than 16 mg/ml, or
  3. Exercise bronchoprovocation test with at least a 20% decrease in FEV1
• Currently on stable dose of daily inhaled corticosteroid for asthma control (i.e., inhaled corticosteroid equivalent to 2 puffs of 44 ug twice per day [176 ug] of fluticasone or greater for 8 weeks or longer prior to study entry)

• Poor asthma control as defined by any one of the following criteria:

  1. Use of beta-agonist for asthma symptoms twice a week or more on average in the month prior to study entry
  2. Nocturnal awakening with asthma symptoms more than once per week on average in the month prior to study entry
  3. Two or more emergency department visits, unscheduled physician visits, prednisone courses, or hospitalizations for asthma in the 12 months prior to study entry
  4. Juniper ACS of 1.25 or greater at the first screening visit
• Absence of GERD symptoms at the time of study entry

Exclusion Criteria:

• Previous anti-reflux or peptic ulcer surgery
• Previous tracheo-esophageal fistula repair
• FEV1 less than 60% of predicted normal value at screening visit and as measured immediately before methacholine bronchoprovocation; methacholine bronchoprovocation will be limited to participants with a FEV1 greater than or equal to 70% of predicted value in accordance with American Thoracic Society (ATS) guidelines
• History of a premature birth of less than 33 weeks gestation or any neonate requiring a significant level of respiratory care, including mechanical ventilation
• Any major chronic illness, including but not limited to non-skin cancer, cystic fibrosis, bronchiectasis, myelomeningocele, sickle cell anemia, endocrine disease, congenital heart disease, congestive heart failure, stroke, severe hypertension, insulin-dependent diabetes mellitus, kidney failure, liver disorder, immunodeficiency state, significant neuro-developmental delay or behavioral disorder (excluding mild attention deficit hyperactivity disorder), or other condition that would interfere with participation in the study
• History of phenylketonuria
• Medications for treatment of GI symptoms (e.g., proton pump inhibitors, H2 blockers, bethanechol, metoclopramide) in the month prior to study entry (intermittent anti-acids are allowed)
• Use of theophylline preparations, azoles, anti-coagulants, insulin for Type I diabetes, digitalis, or oral iron supplements when administered for iron deficiency in the month prior to study entry
• Use of any investigative drug in the 2 months prior to study entry
• Previous adverse effects from lansoprazole, other proton pump inhibitors, or sensitivity to aspartame
• Inability or unwillingness of the legal guardian to provide consent
• Inability or unwillingness of the child to provide assent
• Inability to take study medication
• Inability to perform baseline measurements
• Less than 80% completion of screening period diaries
• Inability to contact by telephone
• Planning to move out of the area in the 6 months following study entry
• Pregnancy