A 45-year-old female developed cytogenetic relapse of chronic myelogenous leukemia 4 years after allogeneic bone marrow transplantation. To induce a graft-versus-leukemia effect, peripheral blood buffy-coat cells were collected from the original marrow donor during 5 rounds of leukapheresis over 3 weeks, and 2.47 x 10(8) cells/kg were infused into the patient. Acute graft-versus-host disease (GVHD) did not develop even after an interval of 50 days from the last donor leukocyte transfusion (DLT). However, karyotypic analysis of bone marrow cells performed on the same day showed an apparent decrease in the proportion of Ph1 chromosome-positive cells (1/20) among all dividing cells, compared with the proportion (13/20) observed before DLT. At the same time, the proportion of red blood cells (RBCs) of donor origin among the peripheral RBCs of the patient, which was less than 10% before DLT, began to rise and reached 100% at 4 months after DLT. The karyotype of bone marrow cells obtained on day 90 after DLT was completely normal. Although chronic GVHD of the buccal mucosa and liver developed in association with pancytopenia on day 71 after DLT, this improved rapidly with oral administration of cyclosporine. The clinical course of this patient suggests that acute GVHD is not a prerequisite for elimination of Ph1-positive cells by DLT.