OBJECTIVES: The use of either prednisolone or low-dose dexamethasone in the treatment of childhood croup lacks a rigorous evidence base despite widespread use. In this study, we compare dexamethasone at 0.6 mg/kg with both low-dose dexamethasone at 0.15 mg/kg and prednisolone at 1 mg/kg. METHODS: Prospective, double-blind, noninferiority randomized controlled trial based in 1 tertiary pediatric emergency department and 1 urban district emergency department in Perth, Western Australia. Inclusions were age >6 months, maximum weight 20 kg, contactable by telephone, and English-speaking caregivers. Exclusion criteria were known prednisolone or dexamethasone allergy, immunosuppressive disease or treatment, steroid therapy or enrollment in the study within the previous 14 days, and a high clinical suspicion of an alternative diagnosis. A total of 1252 participants were enrolled and randomly assigned to receive dexamethasone (0.6 mg/kg; n = 410), low-dose dexamethasone (0.15 mg/kg; n = 410), or prednisolone (1 mg/kg; n = 411). Primary outcome measures included Westley Croup Score 1-hour after treatment and unscheduled medical re-attendance during the 7 days after treatment. RESULTS: Mean Westley Croup Score at baseline was 1.4 for dexamethasone, 1.5 for low-dose dexamethasone, and 1.5 for prednisolone. Adjusted difference in scores at 1 hour, compared with dexamethasone, was 0.03 (95% confidence interval -0.09 to 0.15) for low-dose dexamethasone and 0.05 (95% confidence interval -0.07 to 0.17) for prednisolone. Re-attendance rates were 17.8% for dexamethasone, 19.5% for low-dose dexamethasone, and 21.7% for prednisolone (not significant [P = .59 and .19]). CONCLUSIONS: Noninferiority was demonstrated for both low-dose dexamethasone and prednisolone. The type of oral steroid seems to have no clinically significant impact on efficacy, both acutely and during the week after treatment.