The Port Pirie Cohort Study is the first study to monitor prospectively the association between lifetime blood lead exposure and the prevalence of emotional and behavioral problems experienced by children. Lead exposure data along with ratings on the Child Behavior Checklist were obtained for 322 11-13-year-old children from the lead smelting community of Port Pirie, Australia. Mean total behavior problem score (95% confidence interval (CI)) for boys whose lifetime average blood lead concentration was above 15 μg/dl was 28.7 (24.6-32.8) compared with 21.1 (17.5-24.8) in boys with lower exposure levels. The corresponding mean scores (95% CI) for girls were 29.7 (25.3-34.2) and 18.0 (14.7-21.3). After controlling for a number of confounding variables, including the quality of the child's HOME environment (assessed by Home Observation for Measurement of the Environment), maternal psychopathology, and the child's IQ, regression modeling predicted that for a hypothetical increase in lifetime blood lead exposure from 10 to 30 μg/dl, the externalizing behavior problem score would increase by 3.5 in boys (95% CI 1.6-5.4), and by 1.8 (95% CI −0.1 to 11.1) in girls. Internalizing behavior problem scores were predicted to rise by 2.1 (95% CI 0.0-4.2) in girls but by only 0.8 (95% CI −0.9 to 2.4) in boys.