The outcome of allogeneic haemopoietic transplants including the rate of immune complications for patients with chronic lymphocytic leukaemia (CLL) refractory to or relapsing after chemotherapy with fludarabine was analysed. Fifteen patients with advanced CLL who received allogeneic transplantation were prospectively analysed. All patients had previously received chemotherapy with fludarabine for 3-15 courses; 12 were refractory. The median number of circulating CD4+ and CD8+ lymphocytes at the time of transplant was 0.49 x 10(9)/l and 0.23 x 10(9)/l, respectively. One patient was transplanted from a one HLA-antigen mismatched unrelated donor. Three others received a one or two antigen mismatched graft and 11 had HLA-identical sibling donors. Patients received cyclosporine or tacrolimus in addition to methotrexate or methylprednisolone for prophylaxis of acute graft-versus-host disease (aGVHD). Fourteen patients engrafted; one patient had graft failure, but recovered after therapy with intravenous immunoglobulin. 13 (87%) achieved complete remission (CR). Nine (53%) remain alive and in CR with a median follow-up of 36 (range 3-60) months. None developed visceral graft-versus-host disease. These data compared favourably to published reports in other leukaemia patients and for patients with CLL who received a comparable immunosuppressive therapy but without prior fludarabine exposure. This data indicates that allogeneic haemopoietic transplantation can induce durable remission in patients with CLL refractory to fludarabine and that it is reasonable to delay transplantation until failure of fludarabine therapy. It also suggests that prior exposure to fludarabine may decrease the incidence of severe aGVHD, possibly through its immunosuppressive effects. Further studies are warranted to evaluate this observation.