The frequencies of each type of arrythmia were investigated, by age and sex, among 3056 Brazilian patients with cardiac arrhythmia. Each patient investigated had been tested serologically for American trypanosomiasis and 1013 had been found seropositive for the disease. In general, the seropositive 'chagasic' patients were each likely to be suffering from more types of arrhythmia than the seronegative 'non-chagasic', with means of 1.81 and 1.03 types/patient, respectively. Right-branch bundle blocks (RBBB), ventricular extrasystoles (VE) and left anterior hemiblocks (LAHB) were the commonest cardiac arrhythmias among the chagasic patients, and each of these types of arrhythmia (alone or with other types of arrythmia) was more frequent in the chagasic patients than the non-chagasic. The incidence of RBBB among the arrhythmic varied significantly with age in the non-chagasic patients (increasing with age among both the males and females) but not among the chagasic subjects. When the frequencies of each type of arrythmia and each combination of types were compared, the co-occurrence of RBBB and another type of arrhythmia was almost indicative of American trypanosomiasis (occurring in 30.6% of the chagasic subjects but only 2.6% of the non-chagasic). Similarly, only 0.4% of the non-chagasic patients but 7.4% of the chagasic had RBBB, VE and LAHB concurrently. However, the frequencies of RBBB in isolation (i.e. with no other, concurrent, electrocardiographic abnormality), VE in isolation, or LAHB in isolation were not significantly different in the chagasic and non-chagasic patients.