Background: The purpose of the present study was to test a maternal attachment model of behavior problems in early childhood using phase I data from the NICHD Study of Early Child Care, a prospective study of 1,364 children from birth through sixth grade. Methods: Mothers' and caregivers' ratings of children's internalizing and externalizing problems at age three were regressed separately on a set of fifteen predictors that included security and disorganization scores from 15, 24, and 36 months using hierarchical and logistical modeling. Results: There were three main findings. First, Q-set mother-child attachment security, based on home observations at 24 months, provided the best evidence that attachment was associated with behavior problems, especially above-average levels of problems. Second, insecurity in the 36-month modified Strange Situation predicted mothers' and caregivers' ratings of internalizing problems for boys and girls as well as their ratings of externalizing problems for boys. Third, maternal depressive symptoms predicted mothers' ratings of internalizing and externalizing problems. Conclusions: There are meaningful associations between attachment insecurity and behavior problems as assessed not only by mothers but also by caregivers.