Herpes simplex virus (HSV) was found in the tracheobronchial secretions of 14 of 46 (30%) consecutive patients with the adult respiratory distress syndrome (ARDS). The HSV has not hitherto been associated with ARDS, and most previous reports of HSV in the lower respiratory tract have come from autopsy material. In the present study, the diagnosis during life was initially made by identification of the characteristic inclusion bodies of HSV in bronchial epithelial cells obtained from tracheobronchial aspiration. The presence of HSV in the lower respiratory tract was associated with the need for more prolonged respiratory support and an increased late mortality.