Serious respiratory syncytial virus (RSV) disease requiring hospitalization occurs primarily in infants younger than 12 months. The incidence, risk factors, and clinical features in older children have not been studied extensively. Of 282 children hospitalized at our institution with severe RSV disease during a 3-year period, 62 (22%) were older than 12 months. These 62 older children were matched for sex, onset of illness, and hospital location with 62 hospitalized children younger than 12 months with proved RSV infection. Older children had underlying chronic disease more commonly than younger children (47 of 62 vs 24 of 62). Chronic illnesses in older children included bronchopulmonary dysplasia and/or reactive airway disease (34 of 47), congenital heart disease (9 of 47), gastrointestinal disease (7 of 47), and genetic disorders (7 of 47). Three of the four deaths from RSV infection occurred in older children; all four had underlying disease (three with congenital heart disease and one with biliary atresia). We conclude that children older than 12 months with underlying disease are at increased risk for serious or fatal RSV infection and are not always protected by previous RSV disease. Such older children should be considered candidates for passive or active immunoprophylaxis against RSV infection as such agents become available.