This study is a randomized control trial of full- versus half-day prekindergarten (pre-K) in a school district near Denver, Colorado. Four-year-old children were randomly assigned an offer of half-day (4 days/week) or full-day (5 days/week) pre-K that increased class time by 600 hours. The full-day pre-K offer produced substantial, positive effects on children's receptive vocabulary skills (0.275 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, physical, and socioemotional development. At kindergarten entry, children offered full day 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.