Infants and children suffering from some congenital diseases, SAA and haematological malignancies with a poor prognosis are being treated with an allogeneic bone marrow transplantation (BMT), including a graft from another donor than an HLA-identical sibling. In order to obtain engraftment and to circumvent graft-vs-host disease the pretreatment of the host has to be more immunosuppressive and the graft may have to be processed by either T-cell depletion or stem-cell (CD34) enrichment. One or other may result in graft rejection and severe delay of immunological recovery, giving rise to reactivation of DNA-viruses. Most dangerous in this respect are herpesviruses, especially CMV and EBV, and adenoviruses. Studies in the severely compromised BMT-recipients may throw some light on viral pathogenesis and anti-viral immunity. Between January 1985 and January 1997 90 infants and children were grafted with another than an HLA-identical sibling donor in the BMT-centre for children of the Leiden University Hospital. EBV-B-cell lymphoproliferative disease (BLPD) occurred in 10 and severe c.q. systemic adenovirus reactivation in 8 graft recipients, leading to death in 8, respectively 4 of them. Most important variables associated with an increased risk for lethal reactivations were T-cell depletion of the graft and severe delay of haematological and T-cellular recovery after BMT. Whereas anti-viral chemotherapy is efficacious against CMV-reactivations, if administered pre-emptively, this is not the case against BLPD or systemic adenovirus reactivations. The best option for suppression of the latter reactivations is the prophylactic administration of virus-specific cytotoxic T cells, generated from the donor ex vivo.