In widespread informal gold mining in the Amazon Basin, mercury is used to capture the gold particles as amalgam. Releases of mercury to the environment have resulted in the contamination of freshwater fish with methylmercury. In four comparable Amazonian communities, we examined 351 of 420 eligible children between 7 and 12 years of age. In three Tapaj6s villages with the highest exosures, more than 80% of 246 children had hair-mercury concentrations above 10 peg, a limit above which adverse effects on brain development are likely to occur.