Allogeneic bone marrow transplant patients are severely immunocompromised during the immediate posttransplant period, and the risk for common and opportunistic infections may persist for many months. The role of reimmunization for these patients, however, remains unsettled. We briefly review current concepts regarding the recapitulation of immunity from the totipotential hematopoietic stem cells in the donor marrow. The fact that various components of the new immune system mature at different rates can have clinical consequences with regard to specific infections. Most previously immunized patients become antibody seronegative within a few months after allogeneic marrow transplantation. Adoptive transfer of specific antibody-producing cells from the donor to the recipient has been demonstrated in small clinical trials, and is augmented when both donor and recipient are vaccinated. Passive transfer of immunity is more easily achieved to recall antigens than to neoantigens. Primary immunization requires prolonged antigenic stimulation and mature T-cell function or help from natural killer cells. Most healthy patients generate adequate antibody titers to vaccinations that are given 12 months after transplantation, but the presence of chronic graft-versus-host disease can diminish the response. Currently available vaccines have been evaluated in marrow transplant patients. Protein antigens such as tetanus and diphtheria toxoids are more immunogenic than polysaccharide antigens such as pneumococcal vaccine. The new polysaccharide-protein conjugate vaccines, such as the Hemophilus influenzae type b vaccine, also appear more immunogenic. Inactivated poliovirus vaccine has been used successfully. Relatively few data are available about hepatitis B or influenza vaccines. The literature supports the use of standard vaccines in allogeneic bone marrow transplant patients. However, more data on the optimal methods and timing of immunization are needed. We present guidelines for a reimmunization schedule.