The leading cause of infant hospitalizations by chronological age in months, with percentage of total infant hospitalizations and corresponding 95% confidence intervals, National (Nationwide) Inpatient Sample, 2009–2011. Percentage of total infant hospitalizations for acute bronchiolitis due to respiratory syncytial virus (RSV) was provided for age <1 month, 9 months, 10 months, and 11 months when it was not the leading cause. Among age <1 month, acute bronchiolitis due to RSV was the ninth-ranked primary diagnosis. For ages 9, 10, and 11 months, acute bronchiolitis due to RSV was the second-ranked primary diagnosis but was not statistically different from the first-ranked primary diagnosis.