Acute graft-versus-host disease (GVHD) is the most important complication of allogeneic bone marrow transplantation. We investigated the possibility of predicting severe acute GVHD using plasma interleukin-10 levels in 31 patients who underwent allogeneic bone marrow transplantation. In patients with acute GVHD, the interleukin-10 (IL-10) level increased significantly from the aplastic phase through the leukocyte recovery phase after transplantation (P < 0.05, paired t-test). The ratio of the IL-10 level in the aplastic phase to that in the leukocyte recovery phase was significantly correlated with the severity of acute GVHD (P < 0. 05, t-test), and the incidence of grade III or IV disease was significantly increased (P < 0.0001). Since IL-10 antagonizes various other cytokines that induce acute GVHD, determination of the IL-10 level is equivalent to assessing the total production of cytokines promoting GVHD. The ratio of the IL-10 level in the aplastic phase to that in the recovery phase seems to be useful for predicting the subsequent risk of acute GVHD.