OBJECTIVES: I. Determine the rates of durable engraftment in patients with severe aplastic anemia, myelodysplastic syndrome, inborn errors of metabolism, or inherited hematopoietic disorders refractory to medical management, who are undergoing high dose chemoradiotherapy followed by unrelated cord blood (UCB) transplantation. II. Determine the incidence and severity of acute and chronic graft-versus-host disease in these patients. III. Monitor overall and event-free survival of these patients. IV. Evaluate rate and quality of immunologic reconstitution of these patients. V. Determine whether nucleated cell or progenitor cell content of the graft is predictive of engraftment.