To describe the typical hospital course of infection in previously well infants hospitalized with respiratory syncytial virus (RSV) infection, we reviewed the charts of 196 patients with laboratory-proved respiratory syncytial virus infection in the 1987-1988 respiratory disease season. Eighty-seven of the children had been previously well. Their mean duration of hospitalization was 3.4 days. Previously well infants younger than 6 weeks of age experienced significantly longer hospitalizations and more days of supplemental oxygen and were more likely to require intensive care than were older children. Children older than 12 weeks of age were hospitalized for a mean of 2.5 days and did not require intensive care. Oxygen saturation was measured in the emergency room for 67 of the previously well infants; in 42 oxygen saturation was at least 90% whereas in 25 saturation was less than 90% or infants were receiving supplemental oxygen at the time of measurement. Decreased initial oxygen saturation was associated with a prolonged hospitalization (5.3 vs. 3.2 days, P less than 0.01) and with more days of supplemental oxygen (4.4 vs. 1.5 days, P less than 0.01). We conclude that among previously well infants admitted to the hospital with respiratory syncytial virus infection, infants younger than 6 weeks of age are at increased risk for a prolonged and more severe hospital course than are older children.