Toxoplasma gondii is an opportunistic parasite, which very unusually may cause acute encephalitis in patients undergoing chemotherapy or hematopoietic stem cell transplant. The prognosis is usually dismal also because of late diagnosis, depending on the limited availability of specific diagnostic tools. An early diagnosis allows effective intervention with specific antibiotics, which may provide a chance for cure. We report 2 cases of cerebral toxoplasmosis in which the use of polymerase chain reaction on cerebrospinal fluid allowed a prompt diagnosis and specific therapy, which was followed by clinical response and negativization at follow-up studies of T. gondii genome on cerebrospinal fluid by polymerase chain reaction and by brain imaging.