The problem of thyroid dysfunction in Chagas' disease is discussed on the basis of the functional study of three series of patients with the chronic form of the disease. There was no significant association between Chagas' disease and goiter. No significant difference was found between the average plasma PBI in the groups of patients and that of the control groups. However, the values in the Chagas groups showed a much wider dispersion than those of the non Chagas controls; the difference between the corresponding variances was statistically significant. A loss of efficiency of the mechanisms concerned with the homeostasis of circulating thyroid hormone seems to occur in chronic Chagas' disease. This is interpreted as a possible result of the nervous lesion characteristic of the disease, which might involve neuronal links of homeostatic circuits responsible for the release of TSH.