The records of 545 patients were reviewed for risk factors associated with cytomegalovirus (CMV) infection after marrow transplant. CMV infection occurred among 36% of seronegative patients and 69% of seropositive patients. Among seronegative patients, significant risk factors for CMV infection included positive serology of the marrow donor (relative rate, 2.3) and the use of granulocyte transfusions from seropositive donors (relative rate, 2.5). Among both seronegative and seropositive patients, the occurrence of acute graft-versus-host disease (GVHD) significantly increased the risk of CMV infection (average relative rate, 1.8) and of subsequent CMV pneumonia (average relative rate, 2.6). CMV excretion and viremia were each associated with subsequent pneumonia, but the positive predictive values were low. One-third of long-term survivors excreted CMV at greater than 250 days after transplantation. The only risk factor for late excretion was CMV infection that occurred in the first 150 days after transplantation. In contrast to the effect of acute GVHD on CMV infection, CMV infection did not increase the risk of either acute or chronic GVHD.