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Polychlorinated biphenyls (PCBs) have been suspected for some time of having adverse effects on neuropsychological functioning in humans. While there is evidence of slowing of cognitive function in children associated with exposure to PCBs, the evidence of comparable effects on adults is far less well understood. We report here on the neuropsychological evaluation of 277 Native American adults, ranging in age from 18 to 79, who were exposed to PCBs by way of environmental contamination in the St. Lawrence region of upstate New York. PCB body burden was estimated by 101 PCB congeners and neuropsychological functioning was assessed by a battery of 18 tests. Spline regression models were fitted to the latent variables of memory, motor function, and higher-order executive functioning. After adjusting for age, gender, and education the analyses revealed a threshold effect of PCBs at approximately 2ppb. An age-by-PCB interaction effect was also observed for several variables which suggests that the threshold effect was largely confined to the age range of 40–79 and was not observable in the 18–40-year-old group. Implications of these results are discussed in comparison to previously published similar work with adults and in terms of its potential clinical meaningfulness. [Copyright &y& Elsevier]