of specific anti-HHV-6 antibodies in IgM and IgG classes and viral DNA. Quantitative real-time PCR assay was also used to determine viral load in alloHSCT recipients in plasma samples. Results: All individuals from studied group have not IgM antibodies against-HHV-6 prior transplantation. Specific IgG-class anti-HHV-6 antibodies were detected in 38 of 54 (70%) donors and in 47 of 54 recipients before HSCT (870/o), respectively. High load of HHV-6 DNA (>1x10A6 copies/ml) was detected in plasma samples taken only from one person (1,9%) of the 54 recipients. Conclusions: There is a high frequency specific anti-HHV-6 antibody in studied Polish patients; otherwise CI-HHV-6 was rare detected. Nonetheless, we urge careful observation of individuals with hematological malignances supposed to have CI-HHV-6. Further research on larger study group is needed to determine the clear role of CI-HHV-6 in alloHSCT recipients.