Lymphocyte transfusion from the marrow donor (DLT) is well established as an effective therapy for relapse of CML post allogeneic BMT. Reports thus far have been mostly limited to patients who received DLT from a matched sibling donor. We compared the efficacy and toxicity of DLT in 30 patients who were treated with cells from their HLA-identical sibling (n= 18) or from their phenotypically HLA-matched unrelated marrow donor (n= 12). The overall probability of obtaining a cytogenetic remission was 69%(95% CI: 51–83%) and was not significantly different between the two groups. The disease stage at the time of DLT was the only factor associated with cytogenetic remission by multivariate analysis; patients treated in cytogenetic or molecular relapse (n= 11) were seven times more likely (RR= 7.4, 95% CI: 2.4–22.4, P= 0.0005) to respond compared to patients treated for hematologic relapse (n= 19). There was a trend towards more acute GVHD II–IV in the unrelated donor group (58 vs 39%, P= 0.09), but the probability of developing extensive chronic GVHD was not significantly different (56 vs 39%, P= 0.4). We conclude that transfusion of donor cells from HLA-matched volunteer donors does not appreciably increase the risk of GVHD compared with transfusion of cells from HLA-identical siblings in patients with CML who relapse following allogeneic BMT. Conversely, there is no evidence for an increased graft-versus-leukemia effect after DLT from volunteer donors.