OBJECTIVE: This study evaluated the relationship between prenatal exposure to airborne polycyclic aromatic hydrocarbons (PAHs) and child intelligence. METHODS: Children of nonsmoking black or Dominican-American women residing in New York City were monitored from in utero to 5 years of age, with determination of prenatal PAH exposure through personal air monitoring for the mothers during pregnancy. At 5 years of age, intelligence was assessed for 249 children by using the Wechsler Preschool and Primary Scale of Intelligence-Revised. Multivariate linear regression models were used to estimate and to test the associations between prenatal PAH exposure and IQ. RESULTS: After adjustment for maternal intelligence, quality of the home caretaking environment, environmental tobacco smoke exposure, and other potentially confounding factors, high PAH levels (above the median of 2.26 ng/m(3)) were inversely associated with full-scale IQ (P = .007) and verbal IQ (P = .003) scores. Children in the high-exposure group had full-scale and verbal IQ scores that were 4.31 and 4.67 points lower, respectively, than those of less-exposed children (<or=2.26 ng/m(3)). The associations between logarithmically transformed, continuous, PAH levels and these IQ measures also were significant (full-scale IQ: beta = -3.00; P = .009; verbal IQ: beta = -3.53; P = .002). CONCLUSION: These results provide evidence that environmental PAHs at levels encountered in New York City air can affect children's IQ adversely.