We report 2 patients with plasmacytoid dendritic cell leukemia (pDCL) expressing CD4, CD56, CD33, CD36, HLA-DR, CD123, CD86 and CD83 in the absence of lineage markers (myeloid, B, T or natural killer cells) except for CD33. Culturing leukemic blasts of both cases with IL-3 for 4 days increased the expression of surface molecules associated with antigen presentation, e.g. CD1a and CD40. Leukemic blasts of both cases possessed a considerable level of antigen-presenting ability to allogeneic lymphocytes in mixed leukocyte cultures. Culturing the blasts with IL-3 for 4 days markedly increased allogeneic antigen presenting ability. Combined with data showing evident graft-versus-leukemia effects without graft-versus-host disease in a cord blood stem cell transplanted pDCL case, leukemic cells in pDCL may act as potent antigen presenting cells in vivo, too.