The authors report hematologic and cytogenetic data on 19 patients treated with allogeneic bone marrow transplantation (BMT) for severe hematologic disorders: 8 patients with chronic myelogenous leukemia, 6 with acute leukemia, 3 with severe aplastic anemia, 1 with refractory anemia, and 1 with beta-thalassemia major. Cytogenetic assays were performed on marrow cells before conditioning, 30 days after BMT, and at subsequent times. The authors discuss the role of cytogenetic studies in the evaluation of bone marrow engraftment, leukemic transformation of the graft, and disease relapse.