This study explored the effects of a full-day preschool program on 4-year-old children. The study compared the development of a group of children (N = 403) who attended the preschool program on a half-day basis during the 1999-2000 school year (the last year the half-day program was in place) with the development of a group of children (N = 418) who attended the program on a full-day basis during the 2000-2001 school year (the inaugural year of the full-day program). The instruments used in this study assessed language, academic learning, prosocial behavior, conduct problems, and motor skills, as well as the degree to which parents were satisfied with their children's preschool programs. The results suggest that the full-day preschool program had a positive effect on children's language and academic learning as well as on parental satisfaction with the programming but that the full-day program did not affect prosocial behavior and conduct problems.