Do children learn more in full-day kindergartens than half-day programs? If full-day kindergarten increases learning, are kindergartners in some schools particularly advantaged by their full-day experience? We address these questions with a nationally representative sample of over 8,000 kindergartners and 500 U.S. public schools that participated in the Early Childhood Longitudinal Study-Kindergarten Cohort. More than half of kindergartners experience full-day programs, which are most commonly available to less-advantaged children. Using multilevel (HLM) methods, we show that children who attend schools that offer full-day programs learn more in literacy and mathematics than their half-day counterparts. We also explore differential effectiveness in some school settings.