Cytomegalovirus pneumonia complicated bone marrow transplantation in 75 (63 allogeneic and 12 autologous) of 1136 recipients (Kaplan-Meier incidence 8.8%). CMV pneumonia occurred more frequently in allogeneic (12.4%) than autologous recipients (3.3%). Increased risk for CMV pneumonia was observed in allogeneic recipients who were seropositive (relative risk = 2.9), older age (RR = 1.4 per decade), those conditioned with total—body irradiation (RR = 2.7), who received antithymocyte globulin (RR = 2.9) or T cell-depleted marrow (RR = 2.7) or who had CMV viruria (RR = 4.0) or viremia (RR = 5.9). Autologous recipients were also at increased risk if they were seropositive (RR = 6.1), or developed viruria (RR = 7.0) or viremia (RR = 15.4). Thirteen of 14 untreated patients died without improvement. Prognosis was poor in patients who were ventilator-dependent at initiation of therapy (median survival 17 days), with only 1 long-term survivor. In contrast, patients ventilator-independent at initiation of therapy with ganciclovir and immunoglobulin (n=22) had a median survival of >274 days, with 9 long-term survivors. Ganciclovir alone or acyclovir with immunoglobulin in ventilator-independent patients was less effective (median survivals 80 and 10 days, respectively). Overall, 10 of 75 patients were surviving 10-73 months (median 47) from diagnosis; 9 of these were ventilator-independent at initiation of therapy and received ganciclovir with immunoglobulin. CMV pneumonia was less common, but was severe in autologous recipients, with only 2 of 12 surviving. CMV pneumonia remains a prominent cause of death following BMT. Early therapy with ganciclovir and immunoglobulin before respiratory failure supervenes may improve survival.