Data on the prevalence of positive serologic reactions for Trypanosoma cruzi (a protozoan causing Chagas cardiomyopathy) serve for calculating that a total of up to 74,000 Latin Americans residing in the United States have the chronic form of chagasic cardiomyopathy. The vast majority of these individuals are either undiagnosed, or misdiagnosed as having idiopathic dilated cardiomyopathy or coronary artery disease. Vector transmission of T. cruzi infection is very unlikely to occur in the United States because of variations in biological behavior of local species of insect vectors and because of changes in human living conditions. Transfusion of blood from infected but asymptomatic individuals is considered the most important mechanism of transmission of this disorder in the United States.