Prophylactic transfusion of donor lymphocytes (pDLT) is an attractive form of maintenance therapy after allogeneic stem cell transplantation in patients with high risk of relapse. However, clinical experience is limited, and disease response is often achieved at the expense of severe graft-versus-host disease (GvHD). We here report our data on pDLT in high-risk AML and MDS. Cells were given within a prospective protocol that contained a sequence of chemotherapy, reduced intensity conditioning for allogeneic transplantation, and pDLT (FLAMSA-regimen). For pDLT, patients had to be in CR at least 120 days from transplantation, off immunosuppression for 30 days, and free of GvHD. 22/86 patients alive at day +120 fulfilled the criteria for pDLT. They had been transplanted for refractory or relapsed leukemia (n=9 each) or in CR1 because of unfavorable cytogenetics (n=4). 14 patients had an unfavorable karyotype, 8 with complex aberrations. Reasons for withholding pDLT in 64 patients included cGvHD or prolonged immunosuppression (n=38), refractory or relapsed leukemia (n=15), refusal of patient or donor (n=4 each), a history of grade IV acute GvHD (n=2), and chronic infections (n=3). The median time from transplant to first pDLT was 167 days (range 120–297). Median follow up of transfused patients is 696 days (range 209–1341). Ten patients received 1, 6 patients received 2, and 6 patients received 3 transfusions in escalating doses, containing a median of 1x106, 5x106 and 1x107 CD3+ cells/kg at pDLT 1, 2 and 3, respectively. Reasons for giving less than 3 transfusions were GvHD, relapse or refusal of the patient. Induction of GvHD was the main complication; grade III acute GvHD developed in 1, and chronic GvHD in 7 patients. So far, 5 patients have relapsed despite pDLT. One died of refractory leukemia, whereas 2 achieved secondary CR following adoptive immunotherapy. Two patients are currently under treatment. At present, 18/22 patients are alive and in CR at a median of 423 days post DLT. The current leukemia free survival at two years from first pDLT is 79%. Nineteen patients were complete chimeras at time of pDLT. pDLT converted mixed into complete bone marrow chimerism in 1, but failed in 2 cases. In our experience, pDLT is safe after allogeneic transplantation for high risk AML, when given at low doses and to a selected group of patients. Results are encouraging, and long term survival can be achieved. However, further studies need to define more precisely the contribution of pDLT to the therapeutic effect of the entire procedure.