The association between infection with Trypanosoma cruzi and the development of electrocardiographic alterations and mortality was studied longitudinally for 9 years in a population residing in an endemic area in Castro Alves, Bahia, Brazil. Annual censuses were begun in 1973. At 3 year intervals from 1974 to 1983, the population was examined serologically and subjected to electrocardiography using standardized techniques. Of 1,751 individuals registered during the 10 censuses, 1,541 (88%) were examined serologically at least once. Of 747 individuals in constant residence and originally seronegative, 24 were seropositive upon subsequent evaluation, giving a seroconversion rate of 4.92/1,000 person-years (PY). The overall rate of development of an abnormal ECG was 25.7/1,000 PY for seropositive individuals and 12.5/1,000 PY for seronegative individuals, a relative risk of 2. The 10-14-year-old seropositive group had the highest risk of developing ECG abnormalities (24.1/1,000 PY, relative risk = 3.5). The age-adjusted mortality rate of seropositive individuals was slightly higher than for seronegative individuals (8.9 vs. 7.8/1,000 PY). In sero-positive individuals, mortality was strongly associated with the presence of ventricular conduction defects and arrhythmias. Ventricular conduction defects appeared most frequently in younger individuals. Older individuals presented the highest risk of development of frequent and multifocal extrasystoles.