The implications of low-level lead exposure for children's intellectual and academic performance at school age are uncertain. This issue was investigated in a prospective study of middle-class and upper-middle-class children with low lifetime exposures to lead. A battery of neuropsychological tests was administered at age 10 years to 148 children whose lead exposure and cognitive function had been previously assessed at ages 6, 12, 18, 24, and 57 months. Primary endpoints were Wechsler Intelligence Scale for Children-Revised (WISC-R) and the Kaufman Test of Educational Achievement (K-TEA). Higher levels of blood lead at age 24 months, but not at other ages, were significantly associated with lower global scores on both the WISC-R and the K-TEA after adjustment for potential confounders. Over the range of approximately 0 to 25 µg/dL, a 0.48-µmol/L (10 µg/dL) increase in blood lead at 24 months was associated with a 5.8-point decline in WISC-R Fullscale IQ (95% confidence interval: 1.7 to 9.9, P = .007) and an 8.9-point decline in K-TEA Battery Composite score (95% confidence interval: 4.2 to 13.6, P = .0003). Mean blood lead level at age 24 months was 0.31 µmol/L (6.5 µg/dL; SD: 4.9, 90% percentile: 12.5). Slightly elevated blood lead levels around the age of 24 months are associated with intellectual and academic performance deficits at age 10 years.