This study is a randomized control trial of full- versus half-day pre-kindergarten in a school district near Denver. Four-year-old children were randomly assigned an offer of half-day (four days/week) or full-day (five days/week) pre-k that increased class time by over 600 hours. The offer of full-day pre-k produced large, positive effects on children’s receptive vocabulary skills (0.267 standard deviations) by the end of pre-k. Among children enrolled in district schools, full-day participants also outperformed their peers on teacher-reported measures of cognition, literacy, math, and physical development. At kindergarten entry, children offered pre-k still outperformed peers on a widely-used measure of basic literacy. The study provides the first rigorous evidence on the impact of full-day preschool on children’s school readiness skills.