Preleukemia has generally been treated by palliative measures. Several reports have indicated that cytarabine given as low-dose infusion results in responses, albeit short lived, in a fraction of patients. We have shown recently that marrow transplantation offers a useful alternative. Twelve patients have been treated and followed for a minimum of 1 year. Three were conditioned with cyclophosphamide (CY) only, and all died with recurrent or persistent disease. Nine were conditioned with CY and total-body irradiation; all but 1 had lasting engraftment, and 7 are surviving, free of disease, 16-36 (median 27) months after transplantation. These data show that marrow transplantation can provide successful therapy for preleukemia.