One patient with refractory B cell chronic lymphocytic leukemia (CLL) and another with refractory B cell prolymphocytic leukemia (PLL) underwent bone marrow transplantation (BMT) from HLA-identical siblings. Circulating malignant cells persisted at high levels in the patient with PLL and there was clinical evidence of disease progression soon after transplant in the patient with CLL. Starting 4-5 weeks post-BMT, cyclosporine was tapered rapidly to stimulate immunologic graft-versus-leukemia (GVL) reactions. There was a fall in the number of malignant cells and reversal of organomegaly with the onset of acute graft-versus-host disease (GVHD). Both patients received conventional doses of corticosteroids for GVHD which also may have contributed to disease response to some extent. Total clearance of the leukemic cells from the peripheral blood was seen in both patients, and clearance of the marrow was seen in the patient with CLL. However, both patients died of complications of severe GVHD. We conclude that GVHD may be associated with a GVL effect after allogeneic BMT for refractory chronic B cell lymphoproliferative diseases. Whether GVL reaction occur in the absence of clinically obvious GVHD after allogeneic BMT for CLL remains to be seen.