This cross-sectional study examined the association between blood lead levels and neuropsychological and behavioral development of 139 children (7-9 y of age) who attended school in the southwestern part of Mexico City. A trained psychologist administered an IQ test to 84% of the children, and teachers graded them for agility, socialization, expression, and knowledge. Parents also answered a questionnaire on demographic and socioeconomic variables. Anodic stripping voltametry was used to determine blood lead levels. Regression models were used to determine the best predictors of IQ and teachers' rating scores. The mean blood lead level was 19.4 micrograms/dl (standard deviation [SD] = 7.6), with a geometric mean of 17.8 micrograms/dl (95% confidence interval [95% CI] = 16.5-19.1). Blood lead was the strongest predictor of full-scale IQ, and there was a significant negative trend between blood lead, full-scale IQ, and teachers' rating scores. In this study, children with higher levels of blood lead performed more poorly on psychometric tests and had poorer educational attainment than their counterparts. These results suggest an association between neuropsychological and behavioral impairment and lead exposure.