In the present thesis human Torque Teno Viruses (TTV) were analysed in different clinical materials from children and the quantity of the virus was determined. The results were compared with relevant published data. Five different materials were used for the investigation including plasma, urine, stool, cerebrospinal fluid and liver tissue. In plasma after allogeneic stem cell transplantation the specific genotypes of TTV were determined by sequencing. TTV kinetics was also correlated with immune reconstitution after transplantation. The TTV titre in plasma was evaluated in relation to the number of leukocytes. Moreover, to confirm or exclude viral co-infections, five different viruses were investigated after transplantation to assess the correlation with TTV. Additionally, cell populations from peripheral blood isolated by flow sorting were analysed to assess the site of virus replication.