Using data from the National Institute of Child Health and Human Development Study of Early Child Care and Youth Development, the authors investigated whether there was evidence of intraindividual stability in behavior problems over time as well as whether children with higher levels of behavior problems at 24 months and more rapid increases in behavior problems prior to school entry performed more poorly on 1st-grade tests of cognitive ability and achievement than their peers. Three findings were noteworthy. First, there was evidence of both intraindividual and interindividual variability in behavior problems between 24 months and 1st grade. Second, children with higher initial levels of internalizing and externalizing behaviors at 24 months had lower cognitive ability and achievement scores in 1st grade. Finally, children with more rapid increases in internalizing behaviors over time had lower cognitive ability scores in 1st grade; this association did not exist for externalizing behaviors. Implications for future research are discussed.