The frequency and clinical significance of paraproteinemia in patients receiving hematopoietic stem cell (HSC) transplants were assessed. Of 66 patients with hematologic malignancies, excluding multiple myeloma who received an allogeneic or autologous HSC transplant, paraproteins were detected in 12 patients using immunoelectrophoresis. None of the patients showed paraproteinemia before HSC transplantation. The class of paraproteins most commonly seen was IgG. In 9 of these 12 patients (75%), a paraprotein was detected continuously after HSC transplantation for an average duration of 464 days, while others demonstrated a transient appearance of the protein. Paraproteinemia after HSC transplantation was not related to the stem cell source, (allograft vs. autograft), age, gender, viral infection and graft-vs.-host disease (GVHD). None of the patients developed plasma cell dyscrasia after the appearance of the paraprotein, while 1 patient developed secondary acute lymphoblastic leukemia. These findings indicate that paraproteinemia after HSC transplantation may be caused by an aberrant immune reconstitution after both allogeneic and autologous HSC transplantation. A long-term follow-up of patients with paraproteinemia after HSC transplantation is needed to confirm this finding in a larger series of patients.