Children and laboratory animals exposed to lead or PCBs show deficits in many aspects of attention and executive function that have been shown to be impaired in children diagnosed with ADHD, including tests of working memory, response inhibition, vigilance and alertness. Studies conducted to date suggest that lead may impact both attention and response inhibition, whereas PCBs may impair response inhibition to a greater degree than attention. Low level lead exposure has been associated with a clinical diagnosis of ADHD in several recent studies. Similar studies of PCBs have not been conducted. We speculate that exposures to environmental contaminants including lead and PCBs may increase the prevalence of ADHD.