The mean platelet volume (MPV) was measured in 14 patients with acute leukemia during the course of chemotherapy for remission induction and 21 patients undergoing bone marrow transplantation, in order to assess its usefulness as an indicator for megakaryothrombopoiesis at bone marrow suppression and recovery. In patients with acute leukemia, the MPV fell from 9.7 +/- 0.8 fl to 9.2 +/- 0.4 fl during the myelosuppression, and then it rose transiently to 10.2 +/- 0.5 fl and fell to 9.3 +/- 0.5 fl during recovery of platelet count. Similarly, in patients undergoing bone marrow transplantation, the MPV elevation was followed by the recovery of platelet count. Thus, when engraftment was defined as this MPV elevation, engraftment was confirmed significantly earlier by MPV (19.9 +/- 5.8 days) than by platelet count (24.8 +/- 6.7 days). MPV should be a useful indicator for engraftment or recovery from marrow aplasia in cases of acute leukemia and those of bone marrow transplantation.