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Objective. Exposure to lead at levels encountered by urban children impairs cognitive development. An observational study suggested improvement in IQ when blood lead level fell, but the only randomized trial of chelation showed no benefit in IQ. Methods. We did a new analysis of the data from the clinical trial using change in blood lead level as the independent variable. The 741 children began with blood lead levels between 20 and 44 [micro]g/dL, and were 13 to 33 months old at randomization to chelation or placebo. Blood lead levels were measured repeatedly, and cognitive tests were given at baseline, 6 months, and 36 months follow-up. Results. By 6 months after randomization, blood lead levels had fallen by similar amounts in both chelated and placebo children, despite the immediate drops in the chelated group; there was no association between change in blood lead level and change in cognitive test score. Blood lead levels continued to fall. At 36 months follow-up, in the placebo group only, cognitive test scores had increased 4.0 points per 10 [micro]g/dL fall in blood lead level from baseline to 36 months follow-up and 5.1 points from 6 to 36 months. Conclusions. The improvement in scores in the placebo group only implies that factors other than declining blood lead levels per se are responsible for cognitive improvement; it is possible but less likely that succimer, the active drug, impairs cognition. Pediatrics 2002;110: 787-791; child, preschool; environmental exposure; lead poisoning/blood/etiology; cognition/physiology; chelation therapy., ABBREVIATIONS. TLC, Treatment of Lead-Exposed Children; MDI, Mental Development Index; SD, standard deviation. Prospective data from multiple studies in several countries show that lead exposure insufficient to produce symptoms still [...]