During the period from July 1, 1999 to September 30, 2000, 9 children with severe adenovirus infection were treated at Chang Gung Children's Hospital. The mean age was 22 months (range, 5-50 months). All of them had lower respiratory tract infections, which manifested as lobar or segmental pneumonia and pleural effusion. Eight (88.9%) of the 9 patients required intensive care and 4 of them required mechanical ventilation. Abnormal laboratory findings included leukocytosis, elevated C-reactive protein, anemia, and prolonged prothrombin time and partial thromboplastin time. Extrapulmonary complications included hepatitis (6 cases), encephalitis (3), conjunctivitis (3), periorbital ecchymosis (1), and coagulopathy (2). One patient died, resulting in a mortality rate of 12.5%. Follow-up at 3 months postdischarge, 5 patients (62.5% of survivors) had bronchiolitis obliterans and/or organizing pneumonia. Seven patients were infected by serotype 3 adenovirus, 1 patient by serotype 2, and another by serotype 11. In conclusion, the clinical, laboratory, and radiographic features of severe adenovirus infection may mimic bacterial infection. Rapid progression of the clinical course despite antibiotic therapy and the presence of unusual extrapulmonary symptoms are important clinical clues in the diagnosis of severe adenoviral infection.