This study will compare a single dose of oral dexamethasone to placebo in a multicenter, randomized, double blind trial of infants aged 2 to 12 months with first-time bronchiolitis (defined as wheezing within 7 days of onset). This is given as additional therapy beyond any other routine therapy used at that center. No current standard therapy is withheld, and no additional tests or other treatments are part of the study.The primary hypothesis is that dexamethasone will be more effective than placebo in preventing hospital admission. The secondary hypotheses are that dexamethasone will decrease respiratory scores and possibly the duration of the disease when compared to placebo, and that dexamethasone will be as safe and as well tolerated as placebo.

Bronchiolitis is the most common lower respiratory infection in infants, and the respiratory condition leading to the most hospital admissions in young children. It is also probably the most common serious illness of childhood lacking evidence-based treatment. Evidence both for and against the effectiveness of corticosteroids such as dexamethasone has been published. Editorials, expert reviews, and a recent report from the Agency for Healthcare Research and Quality recommend a study such as this one.

Patients will be drawn from the emergency departments at participating medical centers in the Pediatric Emergency Care Applied Research Network, created by the Emergency Medical Services for Children program and the Maternal and Child Health Bureau of the Health Resources and Services Administration to study health problems of high acuity and high incidence in children.

Comparisons: The primary outcome (hospital admission after 4 hours of ED observation) and secondary outcomes will be compared between treatment and placebo groups.

Inclusion Criteria:

• First attack of wheezing within 7 days of onset
• Age 2 months through 11 months (less than 12 months)
• Disease of at least moderate severity (RDAI score greater than or equal to 6)

Exclusion Criteria:

• Prior adverse reaction to dexamethasone
• Known heart or lung disease
• Premature birth prior to 36 weeks' gestation
• History of prior asthma or bronchodilator use
• Immune suppression or deficiency
• Trisomy 21
• Critical or life-threatening complications of bronchiolitis
• Treatment with corticosteroids within 14 days
• Known active chickenpox
• Exposure to chickenpox within 21 days
• Child sent to ED for automatic admission