• The presentation and subsequent course of respiratory syncytial virus (RSV) bronchiolitis may be atypical and unusually severe when simultaneous infection due to other pathogenic agents is present. During the past two years, nine of the 189 pediatric patients hospitalized with documented RSV infection were found to have the following simultaneous isolates from initial respiratory tract specimens: four adenovirus, four pneumococcus, one cytomegalovirus, and one Pneumocystis carinii. Noted complications attributable to the second pathogen included thrombocytopenia and anemia (cytomegalovirus), hepatitis and disseminated intravascular coagulation (adenovirus), and sepsis and osteomyelitis (pneumococcus). Three of the four patients with RSV and adenovirus died of severe respiratory failure despite mechanical ventilation; two of these patients received ribavirin therapy. Rapid identification of RSV is important but should not be a substitute for more comprehensive viral and bacterial evaluation.