We identified 19 persons with B-cell chronic lymphocytic leukemia (CLL) who received genetically identical twin blood cell or bone marrow transplants after high-dose conditioning. Ten are alive (eight disease-free) with a median follow-up of 89 months (range, 31-171 months); 5-year relapse rate was 50% (95% confidence interval (CI), 26-73%). Estimated 5-year survival and disease-free survival were 61% (95% CI, 37-82%) and 45% (95% CI, 23-68%). In two of four patients tested at 12 and 21 months by polymerase chain reaction no evidence of residual CLL was detected post-transplant. In one recipient who relapsed at 6 years, molecular studies showed a different CLL clone from that detected pretransplant. This clone was subsequently identified in the donor suggesting transfer of occult leukemia at the time of transplant. Genetically identical twin transplants can result in long-term disease-free survival and molecular remissions, these data suggest the potential for CLL control in the absence of allogeneic graft-versus-leukemia effect. The case of leukemia transfer indicates the need for careful evaluation of donors prior to graft collection.