['Bronchiolitis, the most common indication for hospitalization of young children, is associated with subsequent asthma. Blood eosinophilia is associated with increased severity of asthma, but it is unclear if eosinophilia is associated with severity of illness in bronchiolitis. We hypothesized that blood eosinophilia is associated with unfavorable short-term outcomes of bronchiolitis hospitalizations.', 'Data from initial bronchiolitis admissions to our institution between 2010 and 2013 were extracted using Population Explorer software (Explorys, Cleveland, OH). Children were categorized as "CBC-none" (no complete blood count [CBC] data during the first 7 days of hospitalization), EOS-positive (at least one CBC with ≥300 eosinophils per microliter or ≥3% of all leukocytes identified as eosinophils) or EOS-negative (at least one CBC and no eosinophilia). The association between hospitalization duration and maximum absolute eosinophil count (AEC) was analyzed using Spearman correlation. Variables independently associated with prolonged (≥72\u2009hr) hospitalization were identified using stepwise multivariate logistic regression.', 'In 1356 inpatients <24 months with bronchiolitis, median hospitalization duration was 2.46 days and 38.0% had prolonged hospitalization. CBC data were available in 32.4% of subjects: 20.7% were EOS-positive and 79.3% were EOS-negative. Increased maximum AEC was significantly associated with longer duration of hospitalization. Prolonged hospitalization was independently associated with EOS-positive versus EOS-negative children (OR 1.88, 95%CI: 1.12-3.17, P\u2009=\u20090.020). Mechanical ventilation was most common in EOS-positive subjects (24.2% of cases), versus EOS-negative (7.2%) and CBC-none (0.7%) (P\u2009<\u20090.001).'] Is blood eosinophilia associated with unfavorable hospitalization outcomes in children with bronchiolitis?