To evaluate the long-term effects (20 years) of a Chagas control program (CCP) in Venezuela, a prospective serologic evaluation was carried out from 1981 to 1984 on 5771 inhabitants (8%) of Roscio county. This region was selected as a representative area where the national CCP was implemented effectively. Comparison with a serologic survey performed in the same region before the CCP disclosed a reduction in seropositive subjects from 47.8% to 17.1% (p less than .001), most marked amongst children and teenagers from 29.9% to 1.9%, suggesting that transmission of the disease had diminished. Similar seropositivity changes after the CCP were observed nationwide. Because decreased superinfection would also be expected to occur, we tried to ascertain whether the clinical outcome of seropositive individuals living in Roscio county had improved. The mean age of seropositive subjects between both surveys increased significantly from 34.9 +/- 17.3 to 46.7 +/- 15.1 years (p less than .001). Additionally, we examined clinically and obtained electrocardiograms from 775 seropositive subjects. They were classified as asymptomatic (group A, n = 614) or as symptomatic, having mild-to-moderate heart symptoms (group B, n = 99) or having advanced congestive heart failure (group C, n = 62). Their electrocardiograms were compared with those of 923 seronegative subjects collected simultaneously and with published data obtained before the CCP. Comparison of the age-related rates of electrocardiographic abnormalities of seropositive individuals before and after the CCP showed that they did not differ significantly by linear regression analysis, by the Kruskal-Wallis test, or by the normal approximation to the binomial distribution.(ABSTRACT TRUNCATED AT 250 WORDS)