In a study of the outcome of marrow transplantation in patients with advanced thalassemia, 40 patients with homozygous beta-thalassemia who were 8 to 15 years of age (median, 10) received HLA-identical allogeneic marrow after treatment with busulfan and cyclophosphamide. Twenty-eight of the 40 patients were alive and free of disease 260 to 939 days after transplantation, and 2 patients were alive with thalassemia 372 and 1133 days after transplantation. The actuarial probabilities of survival and of disease-free survival at two years were 75 percent and 69 percent, respectively. Ten patients (25 percent) died. Three died of cardiac failure, interstitial pneumonitis, or septicemia within 14 days of transplantation. Three died of infectious complications associated with acute graft-versus-host disease at 46 to 97 days, and two died of infectious complications of chronic graft-versus-host disease at 249 and 290 days. Two patients had transplant rejection and died with marrow aplasia 115 and 192 days after transplantation. One patient had rejection after four months and while the marrow was aplastic underwent a successful second transplantation; the patient was alive without thalassemia 624 days after the first transplantation. The actuarial probability of grade 2 or higher acute graft-versus-host disease in the 32 patients with initial sustained engraftment was 35 percent. Three patients had chronic graft-versus-host disease, which was fatal in two and still active on day 710 in the third. We conclude that bone marrow transplantation can potentially save patients with advanced thalassemia from an otherwise inexorable progression to death from the complications of blood transfusions. The ultimate outcome in this group of patients must await a longer follow-up.