To clarify the clinical significance of the presence of fragmented red cells (FRC) after allogeneic bone marrow transplantation (BMT), we measured the incidence and degree of FRC and their relationships to clinical features. The percentages of FRC (%FRC) were measured in 50 patients on weeks -2, 0, 2, 4, 6, 8, 10, and 12. The %FRC in pre-BMT patients (mean, 0.52%; range, 0.04%-1.56%) was higher than in healthy control subjects (mean, 0.08%; range, 0.02%-0.27%). The highest %FRC (> or = 1.3%) were seen in 2 pre-BMT and 17 post-BMT patients. Eight patients who developed thrombotic microangiopathy (TMA) showed %FRC values that were significantly higher than those in patients without TMA. However, the timing of elevated %FRC was delayed until several days after the onset of intravascular hemolysis and/or a drop in platelet count. Of the patients who did not experience TMA, 5 patients with infection and 4 patients with acute graft-versus-host disease (GVHD) also showed significant elevation of %FRC during the clinical course. Furthermore, multivariate analysis results demonstrated that TMA and infection had a statistically significant effect on the high value of %FRC. These findings indicate that the appearance of FRC is a common phenomenon in patients undergoing BMT and is not a predictive factor for the early diagnosis of TMA, although FRC is one of the main laboratory findings in TMA. Furthermore, an increased %FRC is seen in other post-BMT clinical settings, such as infection and acute GVHD.