Specific IgM antibodies were present in the serum of 2 (8%) patients, while the others were negative (92%). Twenty of the 26 persons (77%) had IgG antibodies against HHV-6 before HSCT and the other 6 (23%) were negative. Of the group of 16 patients who had no detectable HHV-6 during the entire post-transplantation period, 10 (62.5% of the HHV-6-negative patients) had a positive result only for the anti-HHV-6 IgG test, but no anti-HHV-6 IgM, in serum sample taken directly before transplantation, 4 (31.3%) were both IgM and IgG negative, and the remaining patient had anti-HHV-6 antibodies of both classes ( Table 1) .