In this study, active human herpesvirus (HHV)-6 infection were investigated in 39 renal and 9 bone marrow/stem cell transplant recipients. For this purpose, the presence of HHV-6 DNA in patients sera have been searched by nested polymerase chain reaction (nPCR). In addition, HHV-6 IgM and IgG antibodies were performed by micro-enzyme immunoassay (EIA) to detect seronegative patients before transplantation and IgM response in active or primary HHV-6 infection. Active infection with HHV-6 DNA positivity was detected in 5.3% of renal and 22.2% of bone marrow/stem cell transplant recipients. Active HHV-6 infection was found to be related with asymptomatic reactivation, graft disfunction and cytomegalovirus disease in renal transplant recipients, and, fever and graft versus host disease in bone marrow/stem cell transplant recipients. It has been concluded that, the investigation of HHV-6 DNA by nPCR in the transplant sera, was a practical and useful method for the laboratories, in order to diagnose active HHV-6 infection, while HHV-6 IgG antibody detection was also useful for the differential diagnosis of primary infection or reactivation/reinfection, but HHV-6 IgM antibodies has low value to detect active HHV-6 infection.