In this study, I investigated English and Spanish vocabulary growth in preschool-age Latino English language learners (ELLs) over the course of a 12-week vocabulary intervention. My sample contained 138 Latino children in 10 Head Start classrooms, randomly assigned as intact classrooms to one of three experimental conditions: a Spanish-English treatment, an English-only treatment, and a control. The home language of all the children participating in the study was Spanish. I administered a shared reading technique, dialogic reading, as the intervention. Teachers in the Spanish-English treatment read two books each week with their children and introduced the children to predetermined target vocabulary words contained in the books, first in Spanish and then in English. Teachers in the English-only treatment read the same two books and introduced the children to the same words, but all in English. The teachers in the control classroom read the same books in Spanish and English but did not use dialogic reading techniques and did not discuss the target vocabulary words. I assessed children at pretest, at mid-intervention, and at post-test with four assessments: a target vocabulary assessment in both English and Spanish, and a general vocabulary assessment in both English and Spanish. Children in both treatment groups showed greater English vocabulary growth than those in the control classrooms, indicating that dialogic reading does enhance English vocabulary growth in preschool-age ELLs. Additionally, children in the Spanish-English treatment classrooms showed greater English vocabulary growth and greater Spanish vocabulary growth than children in the English-only treatment classrooms, confirming that in this sample of Head Start Latino ELLs the inclusion of the home language in instruction facilitated English vocabulary growth more than English-only instruction, while also supporting growth in the children's home language. (author abstract)