ABSTRACT In immunocompromised patients with disseminated infection, human cytomegalovirus (HCMV) is widespread in the microvascular endothelium of multiple organs. Human umbilical vein endothelial cells (HUVEC) were used in parallel to human embryonic lung fibroblasts (HELF) to recover HCMV from blood samples of immunocompromised patients. Using the shell vial technique, comparable median numbers of p72-positive HUVEC and HELF cells were found with the 26 HCMV-positive buffy coat samples out of 150 examined. Analysis of other clinical samples inoculated as controls revealed, in the presence of highly infected HELF monolayers, either the presence of very few infected HUVEC with urine specimens ( n = 10 samples) or the lack of infected HUVEC with throat washes ( n = 3) or amniotic fluid samples ( n = 2). Thus, peripheral blood leukocytes (PBL) appear essential for primary isolation of HCMV in HUVEC. In this respect, HCMV strains, recovered from clinical samples other than buffy coats in HELF only, could be readily adapted to growth in HUVEC by coculturing PBL from healthy blood donors with infected HELF and then inoculating infected PBL onto HUVEC. Recently elucidated mechanisms of interaction of leukocytes and HUVEC with bidirectional transfer of virus seem to provide the basis for the restriction of HCMV primary isolation in HUVEC to blood samples. However, virus strains recovered from only HELF could be adapted to growth in HUVEC when inoculated with HELF-derived (either cell-associated or cell-free) HCMV strains upon primary isolation. In conclusion, due to the in vitro selection of virus variants provided with both PBL tropism and HUVEC tropism, HCMV recovery in HUVEC is PBL mediated and substantially restricted to blood samples. Lack of HCMV recovery in HUVEC from clinical samples other than blood leads to the assumption that epithelial cells, such as urinary, amniotic, or pharyngeal cells, do not possess adequate adhesion molecules to establish close contacts with HUVEC.