There is a definite need for effective intervention programmes that address the social-emotional, character and healthy development of preschool children. Strong social-emotional skills are necessary for successful transitions to formal schooling and for healthy developmental trajectories. The "Positive Action" ("PA") programme has a long history of effectiveness in schools (K-12) and communities, but has only recently developed lessons for preschool settings. The current study reports the results of the first evaluation of these lessons. Children were randomly assigned to classrooms/instructors who had previously decided to offer "PA" lessons or not. Instructors in both "PA" and control classrooms rated children's behaviour at pretest and immediate post-test. Results suggest that the "PA" prekindergarten lessons are effective at improving children's skills and behaviours across all of the domains that the programme addresses.