Prevent-Teach-Reinforce for Young Children (PTR-YC) is a standardized model of positive behavior support designed to deliver individualized interventions for preschool-aged children with severe and persistent challenging behaviors. A randomized controlled trial was conducted to compare effects of PTR-YC with those of a "business as usual" (BAU) comparison condition implemented over the course of one school semester (approximately 3-4 months). Participants were 169 children in pre-K and Head Start classrooms in two states in the western United States. Mixed ANOVAs revealed statistically significant differences in pre-post scores on the problem behavior and social skills subscales of the Social Skills Improvement System (SSIS) as well as direct observation from video samples of the children's appropriate engagement and challenging behavior, with all comparisons favoring the PTR-YC intervention. Results are discussed in terms of the need for effective strategies for enabling early childhood professionals to implement behavioral interventions with fidelity and effectiveness. (author abstract)