This study evaluated the efficacy of allogeneic bone marrow transplantation (AlloBMT) in eight patients who had chronic lymphocytic leukaemia (CLL) in Richter's transformation. Five patients were in resistant relapse and three others were in sensitive or untreated relapse. Three out of eight patients (38%) were alive and in remission at 14 months, 47 months and 67 months respectively. Two of these three patients, including one with an unrelated-donor transplant and a prior, unsuccessful autologous transplantation, received a non-myeloablative preparative regimen. These data suggest that, compared with conventional chemotherapy, AlloBMT improves the prognosis for patients with Richter's syndrome. Further study of a larger number of patients is needed to confirm these results.