Adenovirus (ADV) is increasingly recognized as a cause of morbidity and mortality in transplant recipients, but ADV pneumonitis has rarely been reported after lung transplantation. The few reported instances of ADV pneumonitis occurred mostly in children immediately after lung transplantation suggesting "primary" infection. We report a fatal case of ADV pneumonitis occurring in an adult, 4 years after unilateral lung transplantation, in whom the premortem diagnosis was not determined. Autopsy revealed severe necrotizing bronchitis, bronchiolitis, and interstitial pneumonitis. Characteristic smudgy intranuclear inclusions, immunohistochemistry for viral protein, in situ hybridization for viral genome, and postmortem lung cultures established ADV as the etiologic agent. ADV can cause fatal, occult respiratory infection in adult lung transplant recipients, remote from transplant surgery.