Neurologic complications are extremely common after bone marrow transplantation and occur in well over half of all patients. Approximately 6 per cent of BMT recipients die as a direct result of neurologic problems. Metabolic encephalopathy, the most common clinical syndrome, is usually due to multiple organ failure. The second most common complication is CNS infection with fungi and viruses. Cerebrovascular disorders are the third most common neurologic problem, and most are related to underlying endocarditis (either infectious or nonbacterial thrombotic endocarditis). Less common neurologic complications include side effects of drugs, recurrence of malignancy, and treatment-induced leukoencephalopathy. Neurologic involvement due to GVHD appears to be limited to rare neuromuscular syndromes. No evidence of CNS involvement from GVHD has been detected.