A study of 666 matched pairs (344 males and 322 females) with positive/negative complement fixation test for Chagas' disease is reported. The pairs were obtained at random by a serological and eletrocardiographical study among rural people of an endemic area in Southern Rio Grande do Sul, Brazil (17 municipalities). Besides non specific ECG alterations, the following abnormalities suggestive of Chagas' aetiology were found: atrio-ventricular complete or incomplete block; right bundle branch block either isolated or in association with left hemiblock; frequent and/or polyfocal and/or bigeminated or trigeminated premature beats; abnormalities in ST and T; eletrically inactive areas. According to these criteria 201 seropositive (30.2%) and 66 seronegative (9.9%) individuals showed abnormal ECG signs. The 20.3% gradient, (21.5% in male and 18.9% in female pairs), of the matched pairs was significant at p < 0,001 level. According to these results, a significant prevalence of eletrocardiografical abnormalities suggesting chagastic cardiomiopathy occurs in seropositive individuals.