Chagas' disease is a prevalent disease in South America that is thought to have an autoimmune etiology. We previously identified human Cha as a new autoantigen recognized by chagasic sera. Those sera recognized an epitope spanning amino acids 120 to 129 of Cha, named R3. In the present study we have used the synthetic R3 peptide for the detection of serum immunoglobulin G antibodies from patients at different stages of Chagas' disease, including a therapeutically treated group. The immunoreactivity with R3 by enzyme-linked immunosorbent assay (ELISA) showed 92.4% sensitivity and 100% specificity for Chagas' disease sera. This sensitivity and specificity were higher than for any other autoantigen described to date. No anti-R3 antibodies were detected in sera from Leishmania-infected or idiopathic dilated cardiomyopathy patients or healthy controls from the same areas. Moreover, anti-R3 antibody reactivity detected by ELISA correlated with conventional serological tests as indirect immunofluorescence and ELISA assays with Trypanosoma cruzi extracts and other diagnostic tests as indirect hemagglutination. The levels of anti-R3 antibodies increased with progression and symptomatology of Chagas' disease. More interestingly, a statistically significant fall in anti-R3 antibody titer was observed in patients treated with antiparasitic drugs. Those results suggest that the presence of anti-R3 antibodies is a highly specific marker of Chagas' disease and that R3 ELISA could be helpful in the diagnosis and monitoring of this disease.