OBJECTIVES: To quantify the burden to the US of RSV associated hospitalizations using a new (October, 1996) ICD-9 code for RSV bronchiolitis and two established RSV codes. METHODS: We conducted a retrospective analysis of two recent federal databases. The 1997 National Hospital Discharge Survey (NHDS) was used to identify the number of discharges among children under age 5 with a primary diagnosis of RSV bronchiolitis (ICD 9-466.11); RSV pneumonia (ICD 480.1) and RSV (ICD-9-079.6); patient demographics; length of stay (ALOS); and expected payment source. NHDS formulas were used to estimate standard errors, normal distributions were assumed for the calculation of 95% CIs. Mean charges were generated from the 1996 HCUP-3 NIS and inflated to 1997 dollars using the Medical Care component of the CPI (1.028). RESULTS: In 1997 an estimated 110,000 RSV-associated discharges occurred among children under age five. RSV-bronchiolitis accounted for 84% [95% CI, 71%-97%] of RSV discharges; while RSV pneumonia accounted for 15% [95% CI, 1%-19%]. ALOS for RSV bronchiolitis was 3.5 days [95% CI, 3.0-4.0] ; 4.3 days [95% CI, 3.2-5.4] for RSV pneumonia; and 2.8 [95% CI, 0.3-5.2] for RSV. Mean hospital charges were $9,572 for infants six months and younger and $9,056 for those under age one. 81% [95%CI, 68%-94%] of children discharged with RSV were under age one. CONCLUSIONS: Previous studies may have underestimated the burden of RSV. Estimated total 1997 hospital charges for RSV in children under age 5 were over $750 million with 75% ($580 million) for RSV bronchiolitis. Medicaid insured 50% and private insurers covered 39% of these discharges. The actual burden may be larger since RSV infections may be undercoded and this analysis was restricted to primary diagnoses.