Challenging behaviors that disrupt the preschool classroom are increasingly common and can lead to poor outcomes in adulthood, such as delinquency, aggression, antisocial behavior, and substance abuse. The purpose of this study was to examine the effects of a preschool-based mental health intervention on challenging behaviors amongst 3- and 4-year old children enrolled in four urban preschools in northern New Jersey. The intervention consisted of a mental health consultant providing a curriculum (Second Step) to all 3- and 4-year old children in the preschools, consultation with teachers, and play-therapy for identified children. The Preschool and Kindergarten Behavior Scales (PKSB-2) was utilized to assess change in behavior between the beginning and the end of the school year and a cost-effectiveness analysis was also conducted. Results indicate improved behavior in all areas--social cooperation, social interaction, social independence, externalizing behavior and internalizing behavior--after 1 year of intervention and additional gains following 2 years of intervention. Further, the cost of providing this intervention was determined to be $900 per child, per year. While this study utilized a scale that yields risk levels based on normed comparisons, there was no comparison group in this study. It is uncertain how much change is related to maturation and other factors. However, this study does suggest that the intervention has merit for further research with this population and shows promise for providing a tiered approach to addressing the various degrees of challenging behaviors in the preschool classroom. (author abstract)