Bone marrow transplantation for myelodysplasia and secondary acute nonlymphoblastic leukemia. Twenty-three patients with primary myelodysplasia (MDS) or secondary myelodysplasia/acute nonlymphocytic leukemia (MDS/ANLL) were treated with allogeneic or syngeneic bone marrow transplantation (BMT). Only one patient was in a chemotherapy-induced hematologic remission. Graft-versus-host disease prophylaxis included methotrexate, methotrexate plus cyclosporine, cyclosporine, or T-cell depletion using one of two anti-CD5 monoclonal antibodies. For patients with primary MDS, the median age was 19 years (range, 11 to 41 years) and the actuarial disease-free survival was 56% +/- 21% (median follow-up, 2 years; range, 0.8 to 5 years). There were three graft failures (two with autologous recovery) and two early deaths. Outcome appeared to be related to French-American-British (FAB) classification. For patients with secondary MDS/ANLL, the median age was 28 years (range, 3 to 16 years) and the actuarial disease-free survival was 27% +/- 13% (median follow-up, 5 years; range, 2.5 to 8.5 years). There were no graft failures, two relapses, and four early deaths. The presence of marrow fibrosis per se did not predict for graft failure (P = .21); however, the use of T-cell depleted marrow in patients with marrow fibrosis resulted in graft failure in three of five individuals. Our results suggest that in patients with primary MDS or secondary MDS/ANLL, BMT should be considered early in the course of the disease, and that attempts at inducing a remission prior to BMT appeared to be unnecessary. In MDS patients with marrow fibrosis, T-cell depletion should be avoided. 