Acute lymphoblastic leukaemia (ALL) with the t(4;11) translocation has a very poor prognosis following conventional chemotherapy. Many patients are offered an allogeneic BMT in first remission. We report on the impact of allogeneic BMT on three patients with t(4;11) ALL in first remission. Median age was 20 years. One patient received marrow from an HLA-identical sibling and the other two from unrelated donors. All three engrafted and none of the patients developed acute or chronic GVHD. Remission status was monitored using a sensitive nested RT-PCR to detect the ALL-1/AF-4 hybrid transcript. All three were PCR-negative at 3 months post-BMT. One of the unrelated recipients died of a fungal infection 4 months post-BMT. The other two are alive and in molecular remission at 21 and 24 months post-BMT. This is the first report of longitudinal follow-up of t(4;11) ALL post-allogeneic BMT by PCR. The early attainment of molecular remission in the absence of GVHD suggests that the conditioning regimen may have been more important than a graft-versus-leukaemia effect in these patients. Follow-up of larger numbers of patients will be required to confirm these preliminary observations.