Cytomegalovirus (CMV) is the most common cause of interstitial pneumonia following bone marrow transplantation. CMV pneumonia (CMV Pn) is particularly worrying after allografts since its incidence and severity are closely linked to graft-versus-host disease (GVHD). In similarly conditioned patients, the risk of CMV Pn is the same after autografts and allografts without GVHD; it is inexistant in bone marrow transplantations between twins. These findings, together with numerous experimental data, make CMV Pn a model of viral pneumonia in which the severity of the pneumonia seems to correlate mainly with an immunological reaction that is toxic for pulmonary cells, and CMV acts as a triggering agent rather than as a direct pathogen. As regards treatment, the ganciclovir-immunoglobulins combination has been very encouraging in the first patients treated, but as its mode of action is uncertain our enthusiasm must be tempered, especially since the results recently obtained in a greater number of patients seems to be less favourable than the initial results. The effectiveness of this costly drug combination, which in practice should be reserved for patients who received allografts or autografts plus pulmonary radiotherapy, deserves a more precise re-evaluation.