We analysed data from 114 recipients of HLA-identical sibling transplants who relapsed and received a second transplant between 1978 and 1989. Twenty-nine patients had acute lymphoblastic leukemia, 46 acute myeloid leukemia and 39 chronic myelogenous leukemia. Median (range) interval between first and second transplants was 15 (1-80) months. Following the second transplant, graft failure occurred in 2%, acute graft-versus-host disease (GVHD) in 27% and chronic GVHD in 21% of patients at risk. Risks of interstitial pneumonia and hepatic veno-occlusive disease were higher after the second than the first transplant. Two-year probabilities (95% confidence interval) of treatment-related mortality, relapse and leukemia-free survival were 41% (30-53%), 65% (53-75%) and 21% (14-30%), respectively. Leukemia-free survival was 7% (2-19%) among patients relapsing less than 6 months after their first transplant, with high rates of both relapse, 77% (49-92%), and treatment-related mortality 69% (46-85%). In contrast, leukemia-free survival was 28% (19-41%) in those relapsing more than 6 months after the first transplant; in this group the probability of relapse was 59% (45-72%) and treatment-related mortality 30% (20-43%). Factors correlated with better outcome included a diagnosis of chronic myelogenous leukemia, relapse more than 6 months after the first transplant, acute leukemia in remission prior to the second transplant and good performance status.