Participants' acquiring an active CMV bloodstream infection at the time of the study will receive ganciclovir by the vein twice a day or a high dose of valganciclovir. Participants will be instructed to stop taking the study drug during the treatment time. After active CMV infection can no longer be detected, the treatment will be stopped. Patients will start taking the study drug and will be monitored for a CMV bloodstream infection until 270 days after transplant. Participants developing symptoms that look like CMV disease (pneumonia, infection of the intestines, eye infection) while on the study drug, will have an examination and lab tests to diagnose the organism. Participants that are hospitalized for CMV disease or for any other reason are asked to notify study personnel.