In an effort to delineate the clinical characteristics of respiratory syncytial virus (RSV) infection in the compromised host, we compared children with bronchopulmonary dysplasia (BPD), congenital heart disease (CHD), premature birth, failure to thrive, and gastroesophageal reflux to previously healthy children. During a four-year period, 262 patients were admitted to the hospital with RSV infection diagnosed by a rapid RSV antigen detection test. Children with BPD or CHD had more hospital days and supplemental oxygen days than the previously healthy group (P less than 0.05). Patients with BPD also had more ICU days, ventilator days, and NPO days, as well as a higher physiologic stability index and therapeutic intervention score than the previously healthy group (P less than 0.05). Premature infants were more likely to present with apnea from RSV (P less than 0.001). Patients with underlying illness tended to be older, although significant difference was demonstrated only for the BPD group (7.0 +/- 5.3 vs. 3.5 +/- 3.3, P less than 0.05). Patients with BPD and CHD had more nosocomial infections than the previously healthy group (P less than 0.0001) and death occurred only in patients with underlying illness. We conclude that previously compromised patients are at risk for more severe and prolonged RSV disease. Earlier diagnosis and therapeutic intervention may be necessary in such patients to improve outcome.