At the invitation of the Brazilian government, the authors conducted a multidisciplinary field study in Goiania, Brazil, 3 1/2 years after an accident involving 137Cs, a radioactive cesium isotope. They compared subjects exposed to low levels of ionizing radiation, subjects who experienced anticipatory stress from fear of radiation exposure, and a nonirradiated control group. Results indicated the presence of chronic stress, as measured by psychological, behavioral, and neuroendocrine indices, in subjects who were exposed as well as in subjects fearful of potential radiation exposure. Anticipatory stress associated with potential exposure to ionizing radiation resulted in a level of stress similar to that from actual exposure to ionizing radiation. These findings have implications for past and future radiation accidents.