Purpose: In this study, the authors evaluated the efficacy of a Spanish-English versus English-only vocabulary intervention for dual-language learners (DLLs) with language impairment compared to mathematics intervention groups and typically developing controls with no intervention. Further, in this study the authors also examined whether the language of instruction affected English, Spanish, and conceptual vocabulary differentially. Method: The authors randomly assigned 202 preschool DLLs with language impairment to 1 of 4 conditions: bilingual vocabulary, English-only vocabulary, bilingual mathematics, or English-only mathematics. Fifty-four DLLs with typical development received no intervention. The vocabulary intervention consisted of a 12-week small-group dialogic reading and hands-on vocabulary instruction of 45 words. Postintervention group differences and linear growth rates were examined in conceptual, English, and Spanish receptive and expressive vocabulary for the 45 treatment words. Results: Results indicate that the bilingual vocabulary intervention facilitated receptive and expressive Spanish and conceptual vocabulary gains in DLLs with language impairment compared with the English vocabulary intervention, mathematics intervention, and no-intervention groups. The English-only vocabulary intervention differed significantly from the mathematics condition and no-intervention groups on all measures but did not differ from the bilingual vocabulary intervention. Vocabulary growth rates postintervention slowed considerably. Results support the idea that bilingual interventions support native- and second-language vocabulary development. Conclusion: English-only intervention supports only English. Use of repeated dialogic reading and hands-on activities facilitates vocabulary acquisition. (author abstract)