To identify and treat patients at high risk of cytomegalovirus (CMV) pneumonia after bone marrow transplantation (BMT), we tested for CMV viraemia weekly, and performed broncho-alveolar lavage (BAL) on day 35 post-transplant in 63 recipients. 36 allogeneic BMT recipients were at a high risk of CMV pneumonia (25 CMV-seropositive recipients and 11 patients receiving marrow from a CMV-seropositive donor). Patients with a positive BAL or viraemia received a 14 d course of ganciclovir or foscarnet. CMV was detected in 29 (46%) of the 63 BMT recipients and excretion of CMV in blood and BAL was significantly linked. However, among the 29 patients who excreted the virus, only 10 (35%) shed CMV in blood and BAL at the same time: 19 patients (65%) had detectable CMV in blood (11 patients) or BAL (eight patients) only. Therefore, on the basis of viraemia or BAL alone, 21/29 patients (70%) and 18/29 patients (60%), respectively, would have received antiviral treatment. BAL increased the CMV detection rate by 13% (8/63 patients) relative to viraemia. With this strategy, the incidence of CMV pneumonia was reduced to 3% in allografted patients. Only two of the 19 autografted patients developed fatal CMV pneumonia. We avoided anti-CMV treatment in 54% of all the BMT recipients. In conclusion, CMV should be tested for in both blood and BAL fluid of BMT recipients at high risk of CMV pneumonia.