To study associations between nutritional status and mercury exposure among communities of the Beni River. Cross-sectional survey of mothers and their children from 15 riverside Amerindian communities in the Beni River valley (Amazonian Bolivia). Hair mercury content (H-Hg) served as a bioindicator of mercury exposure. A total of 556 people were measured. Anthropometric indices of nutrition were calculated after measurements of stature, weight, arm circumference and subcutaneous fat folds. Indicators of lifestyle were collected: subsistence activity, fish consumption and ethnicity. The median of H-Hg was equal to 4.0 g/g (CI 95%: 3.64.4). There existed a significant relationship between H-Hg and nutritional indices in 5- to 10-year-old children (F=12.1; p0.0001) but not in other age groups. Fishing activity, fish consumption and ethnicity had an effect upon the variance of H-Hg. Positive relationships between nutritional status and H-Hg may be related to the high nutritional value of fish. Among women, the relationship between H-Hg and nutritional status was negative (F=7.1; p0.001), but this disappeared when ethnicity and subsistence activity were taken into account. In these Amazonian communities, recommendations aimed at lowering fish consumption to prevent mercury exposure should be balanced against nutritional advantages conferred upon growing children.