We report successful treatment of cerebral toxoplasmosis in an unrelated donor marrow transplant recipient. The clinical diagnosis was confirmed by polymerase chain reaction (PCR) amplification for T. gondii-DNA performed both on cerebrospinal fluid and blood leukocytes. Retrospective testing of stored blood samples demonstrated positive leukocyte PCR signal detected up to 52 days prior to onset of clinical symptoms. This case highlights the value of PCR in the diagnosis and early detection of cerebral toxoplasmosis.