Cytomegalovirus (CMV) infections were studied in 785 bone marrow transplant recipients given intensive cytoreductive therapy. CMV excretion occurred in 24%, viremia in 9%, seroconversion in 40%, and overall active infection in 47%. CMV disease was much less common. Retinitis, enteritis, and pneumonitis occurred in only one, five (less than 1%), and 55 (7%) of the patients, respectively. Allograft recipients were more likely to develop CMV disease than were autograft patients (P = .0001) despite comparable rates of active CMV infection. CMV disease was rare after primary infection in both autograft and allograft recipients (0 and 1%, respectively). In contrast, CMV disease occurred in 16% of seropositive allograft recipients. Among allograft recipients, risk factors for CMV pneumonitis were seropositivity, age greater than 10 years, and acute graft-vs.-host disease, while the use of cyclosporine as prophylaxis against graft-vs.-host disease was protective. Although active infection rates did not decrease, the rates of CMV pneumonitis in allograft recipients during successive years declined significantly (P less than .001).