- The incidence of central nervous system (CNS) complications, their risk factors, and impact on outcome are not well defined in recipients of allogeneic hematopoietic stem cell transplant (aHSCT). We reviewed the medical records of 302 consecutive patients who underwent aHSCT for malignant and nonmalignant hematologic diseases at Princess Margaret Hospital between 2002 and 2005. The cumulative incidences of all CNS complications and posterior reversible encephalopathy syndrome (PRES) at days 100 and 365 were 18% (95% confidence interval [CI]: 14-22) and 23% (95% CI: 19-29), and 6% (95% CI: 4-9) and 7% (95% CI: 5-11), respectively. Seizures occurred in 37% of all CNS events in the first 100 days and 73% of PRES episodes. Female gender and high-dose total-body irridiation (TBI) were identified as independent risk factors for CNS complications in the first 100 days posttransplant. Survival at 1-year was significantly inferior in patients who developed CNS complications within 100 days of transplant (28% [95% CI: 17-41] versus 72% [95% CI: 66-77]) and PRES (27% [95% CI: 10-47] versus 67% [95% CI: 62-73]) compared to those who did not (P < .0001). Death from graft-versus-host disease (GVHD) was more common in patients with CNS complications in the first 100 days (P = .006) and PRES (P = .01) compared to patients without complications.