We describe a patient with recurrent relapses after allogeneic BMT for multiple myeloma who repeatedly went into CR after donor leukocyte infusions (DLI). The first bone marrow relapse, 24 months after allogeneic BMT, was treated successfully with the infusion of 1.2 x 10(8) donor T cells. The second extramedullary relapse, 18 months later with a pleural mass and midthoracic spine process, responded again to DLI, however, only after three courses were given, each with escalating doses of T cells. The pleural mass was treated successfully with radiation therapy after the second DLI but reappeared 3 months later and responded again to the final DLI course with 5 x 10(8) T cells/kg. Nevertheless, graft-versus-host disease (GVHD) did not occur. Retrospective analysis of minimal residual disease in bone marrow aspirates during CR periods using a sensitive quantitative tumor-specific PCR showed that BM tumor cell infiltration persisted. The possible clinical implications of this case report, like maintenance DLI and the aim for molecular remissions, are discussed.