Peripheral blood Ia-positive (Ia+) T cells were enumerated in 52 patients who had received allogeneic or syngeneic bone marrow transplantation for the treatment of acute leukemia or severe aplastic anemia. Twenty-two normal people showed 3 +/- 2% of peripheral blood T cells to be Ia+. During the first 130 days posttransplant, all patient groups showed a moderate elevation in the percentage (mean: 21-26%) of Ia+ T cells, regardless of the type of transplant performed, and regardless of the presence or absence of acute graft-versus-host disease (GVHD). Although there was marked individual variation (range 5-76%), there was a trend towards a decrease in the percentage of Ia+ T cells with increasing time after transplantation. Long-term survivors still showed a small (range 3-20%, mean 10%) but significant elevation in the relative number of Ia+ T cells 1-3.4 years after transplantation, regardless of the presence or absence of chronic GVHD. It is not currently known why Ia+ T cells are found in these patients, but accelerated lymphopoiesis, subclinical infection, and excessive immune stimulation caused by microorganisms or other foreign antigens could be contributing factors.