We studied neurodevelopment in infants from two communities. Children living in the vicinity of tin-ore kilns and smelters - TOKS; n = 51) were compared to children from a fishing village (Itapuã; n = 45). Mean hair-Hg (HHg) concentrations were significantly higher in Itapuã children which received significantly (p = 0.0000001) less mean ethylmercury (88.6 μg) from Thimerosal-containing vaccines (TCV) than the TOKS children (120 μg). Breast-milk Pb concentrations were significantly higher in the TOKS mothers (p = 0.000017; 10.04 vs. 3.9 μg L(-1)). Bayley mental development index (MDI) and psychomotor development index (PDI) were statistically significant (respectively p < 0.0000001, p = 0.000007) lower for the TOKS children only at 24 months of age. Multivariate regression analysis showed that MDI was negatively affected by breast-milk Pb and by HHg. PDI was positively affected by breastfeeding and negatively affected by ethylmercury. Milestone achievements were negatively affected by breast-milk Pb (age of walking) and by HHg (age of talking).