CASE PRESENTATION: Two elderly patients were admitted to our Respiratory Intensive Care Unit for severe pneumonia and hypoxemic respiratory failure. After about 2 weeks’ treatment including invasive mechanical ventilation, broad-spectrum antibiotics and corticosteroids, their oxygenation improved with resolution of lung infiltrates on chest radiograph. On the course of recovery, their oxygenation deteriorated again and lung infiltrates extended without positive pathogenic findings of bacteria and fungi. Unexpectedly, their blood CMV DNA became positive and then they were treated with parenteral ganciclovir for more than 2 weeks. After that, their oxygenation and chest radiograph returned to normal.