Breastfeeding seems to confer an advantage to the infant with regard to neurobehavioral development, possibly in part due to essential nutrients in human milk. However, human milk may be contaminated by environmental neurotoxicants, such as methylmercury. At the Faroe Islands where maternal consumption of pilot whale meat and blubber may well cause a considerable transfer of these neurotoxicants into human milk, 583 infants were followed by district health nurses during the first 12 months after birth. Three developmental milestones that are usually reached between between 5 and 12 months of age, i.e., sitting, creeping and standing, were examined. Infants who reached milestone criteria early had significantly higher mercury concentrations in the hair at 12 months of age. This association is contrary to what would be expected from possible neurotoxic effects of mercury. However, early milestone development was clearly associated with breastfeeding which was again related to increased hair-mercury levels. Milestone development was not associated with indicators of prenatal methylmercury exposure, such as the maternal hair-mercury concentration at parturition. The relationship between early milestone development and high hair-mercury levels in the infant therefore seems to be due to confounding caused by the duration of breastfeeding. No other potential confounder played any role in this regard. This study therefore suggests that, if methylmercury exposure from human milk had any adverse effect on milestone development in these infants, the effect was compensated for or overruled by advantages associated with nursing.(ABSTRACT TRUNCATED AT 250 WORDS)