This randomized trial tested the efficacy of an intensive, four-week summer program designed to enhance low-income children's transition to kindergarten (n's = 60 program children, 40 controls). Administered in four public schools, the program focused on social competence, pre-literacy and pre-numeracy skills, school routines, and parental involvement. Hierarchical linear modeling indicated that the program significantly improved teachers' ratings of (a) the transition to the social aspect of kindergarten for girls (but not boys); and (b) the transition to kindergarten routines for the subgroup of children who had the same teacher for kindergarten as for the summer program. Findings are discussed in terms of practices and policies for supporting children's transition to school.