The association between early lead exposure and later problem behaviors was evaluated prospectively in a cohort of 8-year-old children born during a 12-month period at one hospital. Lead levels in umbilical cord blood (means = 6.8 micrograms/dl, SD = 3.1) and the dentin of a shed deciduous tooth (means = 3.4 micrograms/g, SD = 2.4) provided measures of prenatal and postnatal exposure, respectively. Ratings on the Teacher Report Form of the Child Behavior Profile provided information about children's problem behaviors. Cord blood lead level was not associated with the overall prevalence or nature of problem behaviors. In both crude and adjusted analyses, tooth lead level was significantly associated with total problem behavior scores (approximately 2 points in T score per log unit increase in tooth lead). Significant tooth lead-associated increases in both internalizing and externalizing scores were also observed (approximately 1.5 points in T score per log unit increase). Weaker associations were noted between tooth lead level and the prevalence of "extreme" problem behavior scores. The extent to which these associations reflect residual confounding is uncertain. These data suggest, however, that social and emotional dysfunctions are correlates and may be expressions of increased lead exposure.