In this article, we investigate the effectiveness of INSIGHTS into Children's Temperament (INSIGHTS), a temperament-based preventive intervention, in reducing the disruptive behavior problems of young children from low-income, urban families. Results indicate that children enrolled in INSIGHTS evidenced a decrease in disruptive behavior problems over the course of the intervention, with children with high maintenance temperaments evidencing the most rapid rates of decline. In addition, children in a collaborative version of the program with joint parent and teacher sessions demonstrated more rapid decreases in disruptive behavior than children in a parallel version with separate parent and teacher sessions. Furthermore, high maintenance children in the collaborative intervention evidenced lower levels of disruptive behaviors at the end of the intervention than their peers in the parallel version. Increases in parenting efficacy appeared to be the mechanism through which INSIGHTS reduced child disruptive behavior. (Contains 3 tables.)