Ten patients with preleukemia and life-threatening pancytopenia were treated with marrow transplantation. In all ten cases allogeneic marrow was successfully engrafted. In three patients prepared for transplantation with cyclophosphamide alone, the abnormal cell clone either persisted or reemerged within 6 months of transplantation. The other seven patients were treated with cyclophosphamide plus total body irradiation before transplantation and six of the seven are alive and well without evidence of disease from 7 to 25 months after transplantation. These results suggest that cyclophosphamide alone before marrow transplantation is incapable of eradicating the abnormal clone. Cyclophosphamide combined with total body irradiation appears more effective, although more time is needed for full evaluation of results. This experience also emphasizes the importance of performing chromosome studies before transplantation for pancytopenic states in order to identify cases of preleukemia.