Serum ferritin levels were monitored in nine patients with acute lymphoblastic leukemia (ALL), nine patients with acute nonlymphoblastic leukemia (ANLL), four patients with chronic myelogenous leukemia (CML), three patients with non-Hodgkin's lymphoma (NHL), and three patients with severe aplastic anemia (SAA) undergoing bone marrow transplantation (BMT) for hematologic malignancies or aplastic anemia. Serum ferritin analysis was performed before and after BMT at monthly intervals and/or according to the clinical condition of the patient. Serum ferritin increased considerably during the first 3 months following BMT and then decreased in patients with an uncomplicated course. Ferritin levels in the serum of patients who had undergone BMT decreased gradually when complete remission was achieved, but increased with any clinical complication. Thus, elevation of serum ferritin concentration was predictable for clinical complications and for relapse. Patients with acute leukemia with serum ferritin levels above 400 micrograms/l at time of BMT had a risk of relapse within 1 year, triple that patients with lower ferritin levels. All patients who underwent BMT to treat severe aplastic anemia have completely recovered. Accordingly, following an initial increase after BMT, serum ferritin levels returned to normal and remained so in line with the patients' good clinical condition. The findings indicate that serum ferritin yields useful information in the clinical evaluation of patients undergoing BMT.