Human Herpes Virus 6 (HHV-6) is recently discovered between herpes viruses and it is worldwide T- Lymphotropic virus. It was first isolated in 1986 from the peripheral blood of patients with AIDS and lymphoproliferative disorders. CD46 (MCP) is a human membrane cofactor protein. It has recently been identified as a cellular receptor for HHV-6. This study was done on 47 newly diagnosed cases with acute leukemia (AML and ALL) and Disseminated Lymphoblastic Lymphoma (DLL) who were taken from the National Cancer Institute, Cairo University. They were studied before initiation of any specific therapy. Detection of anti HHV-6 IgG in serum of the cases of AML, ALL and DLL cases as well as 2 control groups; the first group was 24 child ALL cases and the second was 16 adult healthy people to detect the seroprevalence of HHV-6. Flow Cytometric Immunophenotyping was used for detection of percentage of blast cells expressing CD46 of the three groups of patients. The results of patients as regarding HHV-6 IgG seroprevalence showed, 29/29 (100%) were positive for AML group, 7/11 (63.6%) were positive for ALL group and 7/7 (100%) were positive for DLL group. The CD46 expression on the blast cells of patients showed, 21/29 (72.4%) were positive for AML group, 6/11 (54.5%) were positive for ALL group and 4/7 (57.1%) were positive for DLL group.