Frequency and clinical significance of cerebrospinal fluid (CSF) pleocytosis in hemopoietic stem cell (HSC) transplantation were surveyed. Cyclosporine (CSA)- or tacrolimus (FK506)-based regimens were used as graft-vs-host disease (GVHD) prophylaxis in allogeneic HSC transplantation. CSF pleocytosis with or without neurologic symptoms was detected in 12 of 25 patients receiving allogeneic HSC transplants but in none of 11 patients receiving autologous HSC transplants. Of the 12 patients with CSF pleocytosis, only one patient developed leukoencephalopathy later. There was a correlation between CSF cell numbers and trough levels of CSA but not with those of FK506. In patients receiving allogeneic HSC transplants, CSF pleocytosis may be relatively common and may reflect neurologic damage associated with calcineurin inhibitors.