Toxoplasmosis is a rare but often fatal complication that occurs after patients undergo allogeneic hematopoietic stem cell transplant. At our institution, toxoplasmosis was diagnosed in 8 of 301 patients who received stem cell transplants. Disseminated toxoplasmosis with a rapid fatal course was observed in 2 patients. Six patients had cerebral toxoplasmosis diagnosed on the basis of neurological signs and observation of the patients' mental confusion, seizures, and typical lesions (which were assessed by computed tomography, magnetic resonance imaging, or both). Seroconversion of antitoxoplasma immunoglobulin and a discovery of toxoplasma deoxyribonucleic acid in the cerebrospinal fluid (confirmed by use of polymerase chain reaction) were documented in all patients. Treatment consisted of clindamycin therapy (for 2 patients) and of pyrimethamine-clindamycin therapy, sulfadiazine therapy, or both (for 5 patients). Patients showed improvement after therapy, as assessed by clinical and radiological means. Three of 8 patients survive-1 without any residual neurological symptoms and 2 with minimal neurological symptoms.