The aim of this study was to evaluate levels and subclass distribution of pneumococcal antibodies in 40 bone marrow transplant (BMT) and 42 autologous bone marrow transplant (ABMT) recipients during the first year after transplant, and response to vaccination with a polyvalent pneumococcal vaccine. Before transplantation, 35/40 recipients of allogeneic grafts, all 42 autologous BMT recipients and 38/39 donors had adult levels of anti-pneumococcal antibodies of the IgG2 subclass. During the first year after transplantation, antibody levels decreased in 29 BMT patients while 11 retained their pretransplant antibody levels. No change was noted among ABMT patients. In the 8 BMT patients who had chronic graft versus host disease (GVHD), none showed normal levels of anti-pneumococcal antibodies 1 year after BMT as compared to 11/32 without chronic GVHD. Three different response patterns were seen after vaccination of 29 BMT patients who lost immunity with a polyvalent pneumococcal vaccine. Ten patients responded with an increase in IgG2 antibodies, 8 responded with an increase in IgG1 and 11 patients did not respond at all. In the 8 patients with chronic GVHD, none responded with an increase in IgG2 antibodies and 6/8 did not respond at all. The results of this study suggest that chronic GVHD is the main factor contributing to loss of immunity to pneumococci and lack of responsiveness to vaccination with pneumococcal polysaccharides after BMT. Furthermore, the difference in capability, between BMT and ABMT recipients, of retaining anti-pneumococcal activity may explain the clinical experience of severe pneumococcal infections in these patients.