The relationship of infection with Trypanosoma cruzi to ECG abnormalities was studied in a defined population in rural Bahia, Brazil. Of 644 individuals 10 years of age or older who had complement fixation tests for antibodies to T. cruzi and ECGs, 53.7% were seropositive. ECG abnormalities were more common in seropositive individuals than in seronegative individuals, and more common in men than in women. The peak prevalence rate of abnormal ECGs occurred among seropositive individuals between 25 and 44 years of age; in this age group ECG abnormalities occurred 9.6 times more frequently among seropositive individuals than among seronegative individuals. The most common abnormalities were ventricular conduction defects, and right bundle branch block with or without fascicular block occurred in 10.7% of the infected population. PR intervals were longer in seropositive individuals than in seronegative individuals. Ventricular extrasystoles were slightly more common in seropositive individuals. A declining prevalence rate of abnormal ECGs among older seropositive individuals suggested selective mortality due to Chagas' heart disease.