Cytomegalovirus (CMV) has long been associated with myelosuppression. Evidence for this association has been provided from in vitro studies, statistical analysis of clinical studies, informative case reports, and murine models. Reports differ as to how CMV mediates myelosuppression. Some data indicate direct infection of hematopoietic progenitors and their progeny, others indicate that the supportive microenvironment is infected and thereby its supportive function compromised. In this report we review data suggesting that the severity of myelosuppression in patients is associated with particular CMV genotypes identified by variations in the glycoprotein B gene.