This paper reports the results from a randomized clinical trial evaluating an adaptation of the Promoting Alternative Thinking Strategies curriculum (PATHS) for preschool-age children in Head Start. PATHS is a universal, teacher-taught social-emotional curriculum that is designed to improve children's social competence and reduce problem behavior. Twenty classrooms in two Pennsylvania communities participated in the study. Teachers in the 10 intervention classrooms implemented weekly lessons and extension activities across a 9-month period. Child assessments and teacher and parent reports of child behavior assessments were collected at the beginning and end of the school year. Analysis of covariance was used to control for baseline differences between the groups and pretest scores on each of the outcome measures. The results suggest that after exposure to PATHS, intervention children had higher emotion knowledge skills and were rated by parents and teachers as more socially competent compared to peers. Further, teachers rated intervention children as less socially withdrawn at the end of the school year compared to controls.