BACKGROUND Many studies have found a significant inverse association between early exposure to environmental lead and cognitive function in childhood. Whether these effects are reversible when exposure is reduced is not clear. OBJECTIVE To assess the reversibility of the apparent effects of lead on cognitive abilities in early childhood by testing whether declines in blood lead concentrations beyond the age of 2 years are associated with improvements in cognition. METHODS Urban and rural communities surrounding a large lead smelter in Port Pirie, South Australia. METHODS A total of 375 children followed up from birth to the age of 11 to 13 years. METHODS Long-term prospective cohort study. METHODS The Bayley Mental Development Index at age 2 years, the McCarthy General Cognitive Index at age 4 years, and IQs from the Wechsler Intelligence Scale (revised version) at ages 7 and 11 to 13 years. RESULTS Mean blood lead concentrations in the children decreased from 1.02 pmol/L (21 .2 microg/dL) at age 2 years to 0.38 micromol/L (7.9 microg/dL) at age 1 1 to 13 years, but cognitive scores in children whose blood lead concentration declined most were generally not improved relative to the scores of children whose blood lead levels declined least. Changes in IQ and declines in blood lead levels that occurred between the ages of 7 and 11 to 13 years (r= 0.12, P= .09) suggested slightly better cognition among children whose blood lead levels declined most. CONCLUSIONS The cognitive deficits associated with exposure to environmental lead in early childhood appear to be only partially reversed by a subsequent decline in blood lead level.