Sixteen bone marrow transplant recipients with cytomegalovirus pneumonia diagnosed by rapid, non-invasive methods were treated with ganciclovir. Seven patients survived the acute infection and there were five long term survivors. Excellent in-vivo suppression of cytomegalovirus was observed. Marrow toxicity was noted in four patients but was rapidly reversible and not life threatening. Clinical features common to surviving patients included good clinical condition, insidious development of infection and evidence of normal alveolar gas exchange. The fulminant onset of symptoms, radiographic abnormalities and hypoxaemia were characteristic of non-survivors. These results offer some encouragement towards further study of ganciclovir for the early treatment of cytomegalovirus pneumonia. To identify such patients, the use of rapid diagnostic methods and aggressive viral surveillance is recommended. Convincing evidence for the efficacy of this drug will only emerge from randomized prospective studies.