Adenovirus pneumonia, while common in infancy and childhood, is rarely documented but may be fatal in the neonatal period. In regard to the serious outcome and no responsiveness to common anti-viral agents, adenovirus infection should be considered in the differential diagnosis of pneumonia in neonates. We report three cases of fatal neonatal adenovirus pneumonia, all of which were diagnosed by postmortem examination. Two patients were born by cesarean section at 35 or 36 weeks of gestation, and the other was a 5100 gm postmature baby born by vaginal delivery at 43 weeks of gestation. Respiratory insufficiency was detected just after birth or in the immediate postnatal period, and was associated with lethargy and chest X-ray findings of pneumonic infiltration. The postmortem findings of these patients were remarkably consistent and characterized by predominant lung involvement. The lungs showed diffuse massive consolidation with scattered patchy hemorrhage, and histologically revealed multifocal necrotizing alveolitis and/or bronchiolitis, often with hemorrhage. Alveolar lining cells and desquamated cells contained numerous smudge ells and many cells with characteristic inclusion bodies. Electron microscopy revealed that these inclusion bodies consisted of arrays of icosahedral particles of adenovirus. It is unusual that one of the patients, who was born by cesarean section without any evidence of prenatal infection, developed adenoviral pneumonia; this indicates that infection may occur in the immediate postnatal period as well as during passage of the birth canal.