Second-language learners display poorer general language skills in the language used at school than their monolingual peers, which is a concern because general language skills (vocabulary, grammar, language expression, and comprehension) provide the foundation for later academic success. In a randomized controlled trial, we examined the efficacy of an intervention to improve second-language learners' general language skills in their second language. One hundred and fifteen children (mean age = 5.5 years) were randomly assigned to a business-as-usual control group or to an 18-week intervention program conducted by kindergarten teachers during the last semester of kindergarten. The children were assessed at pre-intervention, post-intervention and a seven-month follow-up. The children in the treatment group demonstrated significant improvements on a custom measure of taught vocabulary. The language intervention program also produced effects that generalized to standardized measures reflecting expressive language skills. These findings suggest that intervention programs designed to enhance second-language learners' expressive language in the second language can be successfully implemented in kindergarten settings before school entry.