Adoptive immunotherapy with donor leukocytes has emerged as a promising strategy for the treatment of myeloma recurrence after allogeneic transplantation. 2.9 x 10(8)/kg donor mononuclear cells containing 1.4% CD34+ and 37% CD3+ cells were administered to a 48-year-old patient with non-secretory plasmablastic myeloma relapsing 9 months after a blood stem cell transplant from his HLA-identical sibling. In view of the extensive marrow infiltration and the aggressive behaviour of the disease, the donor cells were preceded by a course of EDAP chemotherapy. There was rapid clinical improvement, and CR was achieved on day 30 post infusion. However, three subcutaneous plasmacytomas showing anaplastic features developed within a few days. These failed to respond to interferon-alpha and continued to grow for 5 weeks in the absence of marrow plasmacytosis or other evidence of systemic disease. Grade 3 acute liver GVHD developed on day 79 which was controlled with immunosuppression. Overt systemic relapse occurred on day 90 as the GVHD came under control. The course of our case suggests highly proliferative malignant cells may escape the graft-versus-tumour effect of immunocompetent allogeneic cells in extramedullary sites subsequently resulting in overt systemic relapse if left untreated. New approaches are needed to deal with the problem of extramedullary disease recurrence.