Background: The use of corticosteroids has been shown not to improve the prognosis of patients with bronchiolitis, but it could be assumed that steroids will reduce inflammation. This study aimed to assess whether corticoid therapy influenced the inflammatory and clinical response of critically ill infants. Methods: Prospective, randomized, double blind placebo-controlled trial of glucocorticoids (GCT) in infants of less than 12 monthswith severe or moderate bronchiolitis. Patients were

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... ed to receive systemic corticoid therapy or placebo. The main outcomes were: a) levels of lymphocyte subsets; b) levels of IL-2, IL-12, and IFNγ as pro-inflammatory factors,and c) levels of IL-4 and IL-10 as anti-inflammatory response. Secondary outcomes related with the clinical response were also analyzed. Results: 97 patients were randomized. Evolution of lymphocyte subsets was similar in both groups. Pro-inflammatory interleukins and interferon decreased, but without differences. Anti-inflammatory interleukins showed a significant decrease from baseline to the end of the study, and IL-10 values were significantly lower (p = 0.046) in the GCT group [1.82 pg/ml (1.2-3.5)] vs non-GCT [4 pg/ml (1.5-6.3)]. GCT group showed a lower time of mechanical ventilation and of hospitalization, but without statistically significant differences. No cases of severe adverse reaction to steroids were detected. Conclusions: administration of systemic GCT did not modify the inflammatory nor the clinical response of patients with severe bronchiolitis, except for IL-10 levels that were significantly lower in the GCT group. This can open a line of investigation about the relation of IL-10 and response to bronchiolitis.Trial registration: Glucocorticoid Therapy Impact on the Inflammatory Response and Clinical Evolution in Patients With Severe Bronchiolitis, Clinical Trials.gov registration number: NCT02571517.Study start date: Nov 2011.