The etiology of infantile lobar emphysema is frequently unknown; however, partial airway obstruction is most often implicated. Although infection and lobar emphysema have been linked previously, their cause-and-effect relationship is unclear. We present the clinical, radiographic, and pathologic findings in a 9-day-old infant with acute bronchiolitis due to respiratory syncytial virus (RSV) and a clinical and radiographic appearance suggestive of lobar emphysema. Pathologically, there was diffuse papillary overgrowth of the bronchiolar epithelium due to RSV infection which produced narrowing of the affected airways and alveolar overinflation. We speculate that in this instance RSV infection was at least associated with the development, and was perhaps the direct cause of lobar emphysema.