The factors that impact upon successful bone marrow transplantation leading to immunologic reconstitution in severe combined immune deficiency (SCID), Wiskott-Aldrich syndrome, and in other lethal congenital immunodeficiencies are reviewed. Evidence is presented that graft-versus-host disease (GVHD) can be abrogated by the depletion of T cells, even from histoincompatible marrow grafts. However, graft resistance or restricted immune reconstitution has been observed with significant frequency. The bases for T cell reconstitution and limitations in B cell humoral immune recovery in the postgrafting period are reviewed, together with emerging evidence that pretransplant cytoreduction might obviate some of these problems.