An experimental study was conducted comparing the effects of dual language, or two-way immersion (TWI) and monolingual English immersion (EI) preschool education programs on children's learning. Three-and four-year old children were randomly assigned by lottery to either a newly established TWI Spanish/English program or a monolingual English program in the same district. Children in the study were from both Spanish and English home language backgrounds. All classrooms in the study used the High/Scope curriculum, and all met high standards for teacher qualifications, ratio, and class size. The TWI program alternated between English and Spanish weekly by rotating children between two classrooms (and teachers) each week. Programs were compared on measures of children's growth in language, emergent literacy, and mathematics. Children in both types of classrooms experienced substantial gains in language, literacy, and mathematics. No significant differences between treatment groups were found on English language measures. Among the native Spanish speakers, the TWI program produced large gains in Spanish vocabulary compared to the EI program. Both TWI and EI approaches boosted the learning and development of children including ELL students, as judged by standard score gains. TWI also improved the Spanish language development of English language learners (ELL) and native English speaking children without losses in English language learning.