Selenium (Se) deficiency is linked with some cardiomyopathies. Its status was determined in 170 patients with chronic Chagas' disease from 2 Brazilian regions (Rio de Janeiro and Belo Horizonte), clinically stratified into groups as follows: indeterminate or asymptomatic (IND); cardiac asymptomatic (CARDa); cardiac symptomatic with moderate to severe heart dysfunction (CARDb); and healthy adults (HA), used for comparison. In most HA, Se levels were normal, excluding an overall Se deficiency. Se was significantly lower in CARDb than in HA, IND, or CARDa patients. This was not associated with a concomitant decrease in activity of glutathione peroxidase. Thyrotropin was normal, excluding iodine deficiency. Se correlated positive and significantly with ventricular ejection fraction (assessed via echocardiography). Asymptomatic children with acute Chagas' disease had normal Se as well as 5 noninfectious cases of cardiomyopathy. Low Se was found in 6 of 10 chagasic patients with digestive megasyndromes. Thus, the decrease in Se in chagasic patients seems to be a biological marker for Trypanosoma cruzi infection and related to the progression of pathology.