BACKGROUND Research suggests that hypertonic saline (HS) may improve mucous flow in infants with acute bronchiolitis. Data suggest a trend favoring reduced length of hospital stay and improved pulmonary scores with increasing concentration of nebulized solution to 3% and 5% saline as compared with 0.9% saline mixed with epinephrine. To our knowledge, 7% HS has not been previously investigated. METHODS We conducted a prospective, double-blind, randomized controlled trial in 101 infants presenting with moderate to severe acute bronchiolitis. Subjects received either 7% saline or 0.9% saline, both with epinephrine. Our primary outcome was a change in bronchiolitis severity score (BSS), obtained before and after treatment, and at the time of disposition from the emergency department (ED). Secondary outcomes measured were hospitalization rate, proportion of admitted patients discharged at 23 hours, and ED and inpatient length of stay. RESULTS At baseline, study groups were similar in demographic and clinical characteristics. The decrease in mean BSS was not statistically significant between groups (2.6 vs 2.4 for HS and control groups, respectively). The difference between the groups in proportion of admitted patients (42% in HS versus 49% in normal saline), ED or inpatient length of stay, and proportion of admitted patients discharged at 23 hours was not statistically significant. CONCLUSIONS In moderate to severe acute bronchiolitis, inhalation of 7% HS with epinephrine does not appear to confer any clinically significant decrease in BSS when compared with 0.9% saline with epinephrine.