We report the first case of Chagas' disease causing a brain mass in the acquired immunodeficiency syndrome (AIDS). The illness occurred in a patient living outside a traditional chagasic endemic area and represented reactivation of long dormant Trypanosoma cruzi infection. A Salvadoran-born resident of the United States with diagnosed AIDS developed hemiparesis due to a brain mass. Histopathologic examination and culture of a brain biopsy specimen established that T. cruzi infection produced the lesion. The patient's epidemiologic history excluded newly acquired trypanosomal infection. Thus, cerebral Chagas' disease must now be considered another potential opportunistic infection in AIDS. This is of particular concern as the pandemic spreads to large Latino populations previously exposed to infection with the parasite.