We reviewed the records and marrow biopsy specimens of 75 patients with leukemic, myelodysplastic, or myeloproliferative disorders to determine whether the presence of marrow fibrosis affected engraftment after allogeneic marrow transplantation. While 28 control patients without fibrosis achieved prompt engraftment, two of 32 patients (6%) with mild and five of 15 patients (33%) with severe fibrosis failed. The rate of myeloid recovery was significantly slower and the dependence on platelet and red blood cell transfusions was significantly longer in patients with severe fibrosis than in patients with no fibrosis. A finding of severe marrow fibrosis should therefore be taken into account when evaluating the risks and benefits of marrow transplantation.