A 28-year-old female patient underwent allogeneic PBSCT from her HLA-identical sister for AML in first CR. CD34+ cells were positively selected from PBPC using immunoaffinity columns. She received 8.0 x 10(6) CD34+ cells/kg and 1.74 x 10(6) CD3+ cells/kg body weight (BW). The patient developed acute GVHD III and mild limited chronic GVHD. Thirteen months after transplantation severe thyrotoxicosis requiring plasmapheresis occurred. Immune thyroiditis was confirmed cytologically by lymphocytic infiltration in a fine needle aspirate and by elevated thyroid-Ab-titers. The patient's donor had received thyroid hormone substitution for 10 years for hypothyroidism. The most probable cause of immune thyroiditis after allogeneic BMT is the transfer of antithyroid donor lymphocytes. These lymphocytes can also be transferred with a CD34+ selected peripheral stem cell graft. The transplantation of lymphocyte-depleted autologous bone marrow or PBPC grafts after myeloablative treatment is increasingly considered as potential treatment of severe autoimmune diseases. This case demonstrates that even low numbers of lymphocytes are capable of transferring autoimmune disorders.