Chagas disease is an infection caused by the parasite Trypanosoma cruzi. Reduviids (i.e., "kissing bugs") transmit the parasite through infected feces. T. cruzi also can be transmitted congenitally and through blood transfusion or organ transplantation. The infection is lifelong if left untreated; the majority of infected persons are asymptomatic, and their disease remains undiagnosed. Although routine serologic testing of organ and blood donors is performed in areas of Latin America where Chagas disease is endemic, no T. cruzi screening test is licensed in the United States. However, seroprevalence studies using research tests have documented the presence of T. cruzi antibodies in U.S. blood and organ donor populations. This report describes two cases of acute Chagas disease in heart transplant recipients reported by two Los Angeles County hospitals in February 2006. In the United States, one previous report documented T. cruzi transmission through solid organ transplantation, in which three organ recipients were infected.