Respiratory syncytial virus (RSV) is a major cause of lower respiratory tract infections in infants and young children and the leading cause of hospitalization in infants aged <1 year. We examined trends in RSV hospitalization (RSVH) among infants from 1998 to 2006, using the United States (US) National Hospital Discharge Survey (NHDS) database. RSVH was defined by the International Classification of Diseases, 9th Revision, Clinical Modification (ICD-9-CM) codes 079.6 (RSV), 466.11 (acute bronchiolitis due to RSV), and 480.1 (pneumonia due to RSV). Age at the time of hospitalization was determined using NHDS birth records; RSVH rates were analyzed for infants grouped into three age cohorts (<3 months, 3 to 6 months, and >6 to <24 months). Trends in hospitalization rates were evaluated using linear regression. Relative rates (RR) and 95% confidence intervals (CI) were computed to compare average RSVH rates for infants across age-specific groups. The annual proportion of RSVH by age group was also calculated. Approximately 1.1 million (90,000-147,000 per year) RSVHs in predominantly term children aged <24 months were analyzed. Compared with children aged >6 to <24 months, rates for RSVH were significantly higher among infants aged <3 months (RR, 7.38; 95% CI, 7.35-7.41) and infants aged 3 to 6 months (RR, 5.28; 95% CI, 5.26-5.29). The proportion of RSVH in the first year of life was lowest among infants aged <1 month (9%). [corrected] The greatest proportion of RSVH was observed in children aged 3 to 6 months (14%-23% RSVH per year; chi-square P<0.0001). When the definition of RSVH was expanded to include unspecified hospitalizations for acute bronchiolitis, similar results were observed. RRs were highest among the <3- month and 3- to 6-month age groups. The highest proportion of RSVH was among the 3- to 6-month age group. Analysis of the impact of RSV season, clinical practices, and other factors on these trends is warranted.