Twenty allogeneic bone marrow transplant patients were treated with an additional dose of donor cells (boost dose) for graft failure (n = 7), partial graft failure (n = 11) or extensive hemolysis caused by remaining recipient cells producing anti-erythrocyte antibodies (n = 2). Donors were in 12 cases HLA-identical siblings, three mismatched related donors and five unrelated donors. Cell source was in 13 cases bone marrow and in seven peripheral blood progenitor cells. Median time from BMT to booster dose was 3.4 months (range 0.7-59.3). Median infused cell dose was 2.4 x 10(8)/kg patient (range 0.5-19.0). As GVHD prophylaxis most patients were already receiving different combinations of cyclosporine, prednisolone and methotrexate. No preparative treatment was given prior to boost in 16 patients; four received ATG. After boost, 11 patients developed acute GVHD, six grade I, four grade II and one grade III. Except for one patient, acute GVHD after boost was less, or the same grade as after BMT. Six patients developed chronic GVHD, three limited and three extensive. Five patients died within 30 days of the boost. Nine of 15 (60%) evaluable patients became transfusion independent within 30 days and three more within 60 days. Causes of death were: infections six (IP four, pneumonia two), relapse three; and GVHD three. Three out of five patients transplanted with unrelated marrow suffered from severe immunological reactions and died 2-3 months after the boost dose. Patient survival 1 and 3 years after boost was 55% and 43%, respectively. Among patients with hematological malignancies, leukemia-free survival at 3 years was 41%.