Early trials of allogeneic marrow transplantation for homozygous thalassemia were disappointing in patients older than 16, with four of six patients dying early of graft-versus-host disease-related complications, one patient dying at 9 months of infection due to graft failure, and one dying at 6 years of recurrent thalassemia. Three classes of risk could be identified in analyses of results of transplantation in younger patients using the criteria of degree of hepatomegaly, the presence or absence of portal fibrosis, and a history of adequate or inadequate chelation therapy. Patients for whom all three criteria were adverse constituted a very high risk group (class 3) for marrow transplantation. On the basis of these analyses, a conditioning regimen was designed that yielded superior results for class 3 patients under 17 years of age. Most patients older than 16 years presenting for transplantation have disease characteristics that place them in class 3 and, because of the improved results with the new class 3 regimen in younger patients, a study was designed to treat patients older than 16 years using treatment regimens assigned on the basis of disease class. Twenty patients were treated using this protocol and, with a minimum follow-up of 9 months, there have been three early deaths, one patient has recurrent thalassemia, and 16 patients are alive disease-free. The actuarial probabilities of survival, disease-free survival, and rejection are 0.85, 0.80, and 0.05, respectively, with a survival plateau extending from 6 months to 3 years. Marrow transplantation is a reasonable option for adults with progressive thalassemia who have suitable donors.