The human cytomegalovirus (HCMV) is a major pathogen in immunocompromised patients. Both, primary infection and reactivation of latent virus can cause disease. Peripheral blood leukocytes (PBL) most likely play an important role in viral persistence and dissemination of infection. However, an open question has been whether HCMV actively replicates in PBL in vivo and whether the progenitor cells in the bone marrow are also infected. Previous studies on this issue are controversial. Here we summarize data on the tropism of HCMV for mature leukocyte populations as well as bone marrow progenitor cells during HCMV viremia. All cell populations were highly purified by a fluorescence activated cell sorter (FACS) and analyzed by PCR for the presence of viral genomic DNA. Moreover, mature leukocyte populations were investigated for mRNA expression of regulatory and viral structural proteins. We could show, that HCMV DNA was detected most frequently in granulocytes and monocytes as well as in CD34⁺ progenitor cells of immunosuppressed patients. Viral mRNA expression was found in granulocytes, monocytes, and lymphocyte fractions. In contrast, no HCMV DNA was found in healthy, seropositive individuals.